This is a call for solidarity after the building of the Wall in Berlin.

In the 1950's the Bundestag met in Berlin, expressing the political will, that Berlin (West) was a part of the Federal Republic of Germany (West Germany).

After the Khrushchev's Berlin ultimatum of the 27th of November 1958 this was discontinued.

The shown signs Bundestag nach BERLIN are a call to resume the occasional meetings of the Bundestag in Berlin to make the point clear that military actions to permanently divide the country will not be accepted.

Klaus Schütz (in German), who at the time was a member of the Bundestag, wrote an article in 1963 (as special representative of Berlin to the Federal Government ) going into details why this matter was of great importance to the city (quoted below). He later became Mayor of Berlin between 1967-1977.

In this manner they also wanted to confirm the Federal Republic's claimed of an exclusive mandate for all of Germany and did not recognize the German Democratic Republic (GDR, East Germany) as a state.

East Germany, on the other hand claimed the whole of Berlin (four occupied sectors) for itself.

Officially, the city of Berlin was outside the 4 Zones of occupation and had a special status of its own.

De Facto the Soviet Sector was a part of East Germany.

For the 3 Western Sectors, this was more programmatic due to the geographic situation (an enclave of East Germany).

Strategically it was better for the 3 Powers to retain all occupation rights in the 3 Sectors, since under this condition one would only have to deal with the Soviet Union as the 4th Power (and to ignore the existence of East Germany).

Politically, Berlin (West) was considered a part of the Western block.

This lead to a cat and mouse game that (more or less) ended with the Four Power Agreement on Berlin of the 3rd of September 1971.

On the 7th of April 1965 a Bundestag Plenary Session took place in the Kongresshalle

• today Haus der Kulturen der Welt, nicked named: "The pregnant Oyster"

It was also the last one to take place before the unification in 1990.

Berlin - eine "alliierte Stadt"? | ZEIT ONLINE (in German)

• Author Klaus Schütz (in English), published 1963-05-17, Mayor of Berlin 1967-1977

Berlin is first and foremost a German city. This statement has nothing in common with the patriotic pathos that is often underlaid by such explanations. "Berlin is first and foremost a German city" - that means: Berlin is part of the Federal Republic of Germany. But this means: beyond the bare will to exist, one has to give one's people the opportunity to keep this city viable and to develop it further with the assurance of the future. The status of Berlin thus includes the "established" ties that are indispensable for the viability of the city within the framework of the Federal Republic.

West Berlin, with the approval of the three powers, with the knowledge of the Soviet Union and the will of the affected population, has grown together economically, financially, legally and politically with the rest of the Federal Republic over the past 15 years. These grown bonds are vital. The goal of the German Berlin policy must be to maintain these bonds. And they are to be strengthened where this is possible in international legal and security terms.
...
Bundestag Plenary Sessions as a living expression of the city's close ties to the Federal Republic were regularly held once a year from 1955 to 1958. The right to do so - apart from some concerns in their own ranks - was never seriously contested by anyone.

Only when the precariousness on the basis of Khrushchev's Berlin ultimatum of November 1958 took over, the exercise of this right was waived again for the first time and later at each new attempt. In 1959, the Federal Assembly was convened to elect the new Federal President to Berlin. After an inglorious period of tug-of-war behind the scenes - the resistance at that time did not come from the Allied side - the President of the German Bundestag made his decision.
...

That can mean two different things:

1. A call to relocate the parliament seat from provisional Bonn to the old capital: like it was done after 1990. Small problem with that: The Reichstag was in ruins. So, did they want to build a new house of parliament as well? Likely not in this context. West-Berlin was just a Bundesland, not the capital, and the allies didn't even like that status and demanded from the West-Berlin parliament to tone down:

   "postpone" Article 1 of the Berlin Constitution. The article proclaimed: "Berlin is a state of the Federal Republic of Germany."

   Relocating the Bundestag was just not an option at the time. That the sign would mean anything like that would make it a completely unrealistic demand. But as this is a a protest demonstration, it would perhaps not the first unrealistic sign to show up on such an event?

2. A call for members of parliament to show up in Berlin. Joining in the protest against the building of the wall. That is the more likely explanation, as some politicians protested this, but at the time protest was often seen as too risky. It was seen that interfering in Berlin in any way with Soviet/East German interest was possibly provoking World War III. Even more important: everyone was surprised about this actually happening, since really nobody ever wanted to build a wall.

Option 2 is actually hinted at by two other signs held up in that photo, right next to the others:

One reads: "Where is the chancellor? Is he playing boccia?"

This alludes to the 'official' inaction of the Adenauer government (him being famous enough for this sport that Wikipedia makes him an unnamed Italian:

The next reads: "What else needs to happen for something to happen?"

On 15 June Ulbricht denied that a wall was being planned, on 13 August 1961 they started building the wall.

Clay and Vice President Lyndon B. Johnson arrived at Tempelhof Airport on the afternoon of Saturday, 19 August 1961.

Then there were petty party politics at work as well. The Berlin mayor at the time was Brandt, also nominated for chancellor at the time.

During his election campaign appearance on 14 August in Regensburg, Adenauer, who was under pressure after the beginning of the blocking measures, committed a gaffe and called out "Mr. Brandt alias Frahm". Opinion polls show that these attacks cost Adenauer sympathies among the population. Election campaign instead of a determined joint appearance in coping with the Berlin crisis? That was not what the public wanted. Even a letter to Brandt on 31 August contained no sign of regret. The roles in the Berlin question were clearly divided and came to a head during the election campaign: the acting governing mayor against the dithering Federal Chancellor.
_{-- Hanns Jürgen Küsters & Ulrike Quadbeck: "Konrad Adenauer Bau der Berliner Mauer 13. August 1961"}

So anything resembling positive action was unlikely to come from CDU politicans, just to not empower the SPD mayor and election candidate any more.

The biggest newpaper opened their day with the following headline:

16 August 1961
The newspaper "Bild" has the headline: "The East is taking action - what is the West doing? The West is doing nothing! US President remains silent ..., Macmillan goes hunting ... - ... and Adenauer scolds Brandt."

But more important was the un-sovereignty of any German politician. They could not do really anything without allied approval, especially not in Berlin, except to cause unrest. That was the last this tense situation needed.

While the people in the picture obviously wanted solidarity from the Bonn parliament and more of its members showing up in Berlin for protest, a huge list of reasons prevented that from happening.

A case could be made for option 1 in the sense of option 2 though: not only individual members of parliament showing up, but the whole institution meeting officially for a plenary session as the Bundestag held four such debates officially in West-Berlin:

[The allied powers] accepted until the Bundestag session last week that - the Bundestag visited Berlin four times between 1949 and 1958 (October 1955, 1956, 1957 and 1958);

And in 1955 the first of these meetings was held under the following preamble:

The Bundestag wants to announce that it feels responsible for the fate of this city. The freedom of Berlin and the reunification of our fatherland are a matter of course and a decisive goal of German politics.

One could easily see that such attitude didn't meet the biggest approval all around in minds of all the allies. Especially not if one considers that the then president of the Bundestag Eugen Gerstenmaier said:

If the reunification of Germany and - I might add - the restoration of Berlin as the capital of the Reich were to be placed under the free will of this House, both would have long since been resolved and completed.

These occasional meetings of parliament in West-Berlin was indeed abandoned after 1958, not contemplated in 1961 but when it was repeated in 1965, the allies, mainly the Soviets didn't like it, and tried mightily to interfere, drowning out speeches with overflying fighter jets!

in short

West German politicians were seen like any other Western politicans: doing nothing to prevent the city being split with a wall. Bundestag is therefore meant as members of parliament, come here and look what your inaction is doing to our city. Come and help us. Protest signs aren't always calling for meaningful or realistic options…

So the context is: August 13, Wall is built, Berliners don't like it, can't do anything about it. 300000 go on a protest march on August 16. Chancellor Adenauer and other politicians are expected 'to help' but don't even show up. Adenauer addresses Berliners immediately via radio, only 'to stay calm', then via TV on August 19, yet himself only comes on August 22.
Why? Situation is extremely tense. Adenauer is calling the shots within West-German politics, but allies still hold sway over that and especially controlling effectively everything concerning Berlin.

Regarding the question whether or not the Bundestag should come together once in a while in West-Berlin, Adenauer said this in 1963:

Bailey: I think a lot of people believe that. I think a lot about symbols. Symbols mean a lot to Germany in the current situation. I have heard a lot about Americans, especially in the last three years, and there is even an ongoing discussion among Americans, even among English people, about the question: Why don't they do it? I am referring to the Bundestag in Berlin. Why does the Bundestag no longer meet in Berlin?

Adenauer: I will give you the answer immediately - by the way, ten committees will meet in Berlin in one fell swoop next week.

Dr. Ackermann: 13! The number has even increased.

Adenauer: This is going to be a very big deal. But let us understand our situation: we do not know American policy, the policy of détente – I can no longer bear the word at all!

Bailey: (laughs) Me neither. But you know, Dr. Adenauer, Dean Acheson gave a lecture yesterday ––

Adenauer: I read that. I think that is also the most sensible thing to do, and it seems that the Americans are now changing their minds a little. Unfortunately, Dean Acheson has no influence on the administration.

Bailey: He didn't. But he may have regained ground recently. I have heard, for example, that a few weeks ago American Ambassador McGhee said here in Bonn that the German cause accounted for approximately 1% of the total extent of American policy. But he made quite a correction last week.

Adenauer: That was a very ill-considered speech!

Bailey: I have heard, for example, from American politicians and diplomats that the Germans could - this has been said quite privately – for example make a political issue out of the desire to bring the whole Bundestag, that is to say in plenary, to Berlin. These diplomats recently told me that this would most probably not suit the Allies. But one could force it, one could turn it into a political issue, whether an obstruction on the part of the Allies would be worse, or take the risk of letting the Bundestag go to Berlin.

Adenauer: Should we then change the attitude of the three Western Allies - at least with two, I mean now with the British and the Americans - from the roots? You have to consider that for once! You spoke of isolationism yourself. And what could be more natural than for young Americans, after all, to see Europe as nothing more than a burden? That is why Foster Dulles - and I have always encouraged him to do so - told every American: you are defending your country here. After all, in foreign policy only the interests of one's own country are decisive, and nobody pursues foreign policy out of mercy. It is important to make it clear to the American people that America is being defended in Europe.

Bailey: But on the other hand, if the Americans or the British, the allies par excellence, have the impression ...

Adenauer: They don't have that at all, they only say that because they themselves have a certain - it's unpleasant for them to say now: "Away with the stuff. That's why they say the Germans don't want it at all. Ask one of the gentlemen here what their opinion is, whether we want reunification or not. Do you know that Bismarck has already suffered from the fear that the Russians were in Dresden? This whole colossus is standing before us, and it will continue to roll unless a decisive hold is offered by the leading great powers. What do you do? We are doing everything.

Bailey: I also consider the Wall to be perhaps the greatest provocation that there has been in modern times.

Adenauer: And you are Americans! I would say that you should also see it as a slap in the face.

Bailey: I am now thinking of the young Germans. In the last six weeks I have done nothing else, so to speak, but travel around asking politicians and other key people what they actually think about the reunification issue, and they are fully committed to it.