What is the meaning of this excerpt from Foundation of the Christian Faith, by Karl Rahner?

The object of that passage is Holy Scripture.

The short answer to your question is that Rahner is addressing an argument somewhere else about differences between the early church and the modern church. I had to go from the inside out to get to his flow of reasoning, but the bottom line that he presents is that Scripture, even though it grew from the church (and canon grew from that) rather than being there at the Church's origin, is a norm shared by the modern church and the original / early church since the church (then and now) share the same essential nature.

Whether he is addressing something in his own work, or is answering a position or argument external to his book, is unclear from that brief passage.

Rahner's passion for philosophy (IIRC he studied Kant in considerable depth, was a student of Martin Heidegger, and taught philosophy for some years) informs his style.

This passage comes from a book by Karl Rahner called Foundations of Christian Faith. (The original title is Grundkurs des Glaubens). In the translation by William V. Dych (New York: Crossroad, 1978), the passage can be found on page 371.

What Rahner is asserting generally in this passage is that the Bible is essentially the Church’s book: it was produced by the Church for its members.

Let us parse Rahner’s rather dense prose. My interpretations follow each block quote.

We regard Holy Scripture as the church's book, the book in which the church of the beginning always remains tangible as a norm for us in the concrete.

The Bible belongs to the Church. Through the Bible, the original Church (the one founded by Christ on the Apostles) makes itself present and tangible for us today. As such, the Bible also normative: that is, it instructs us as to what we need to believe and how we need to behave.

Indeed it is a norm which is already distinguished from those things which are found in the original church but which cannot have a normative character for our faith and for the life of the later church.

The Bible transmits to us the essential norms of the Faith, not those that are specific to the time and place of the early Church. (E.g., in the early Church, some communities judged that it was ethical to eat meat sacrificed to idols, because the idols are just superstition anyway, whereas others judged that such meat should not be eaten so as not to mislead those who are weak in the faith. The concrete practice—eating the meat or not—is not an essential norm, because the situation is particular to time and place.)

If the church in every age remains bound to its origin in its faith and in its life;

Fairly clear as written. Clearly, despite the conditional formulation, Rahner is asserting that the Church in fact really is bound to its origin in its faith (beliefs) and life (morals).

if the church as the community of faith in the crucified and risen Jesus is itself to be in its faith and in its life the eschatological and irreversible sign of God's definitive turning to the world in Jesus Christ, a sign without which Jesus Christ himself would not signify God's irreversible coming into the world and would not be the absolute savior;

By establishing the Church, God left an indelible (“irreversible”) sign of his Incarnation (his “turning” to the world, in Rahner’s terminology). The Church is also an indelible sign of the glory to come (an “eschatological” sign).

Jesus himself (through the Incarnation) is a sign of God’s presence in the world.

In fact—asserts Rahner— Jesus would not be such a sign, indeed would not be the “absolute” (i.e., unique, supreme) savior of the world, if it were not for the Church.

(I must say that I find this last affirmation questionable. God is perfectly capable of saving people however He wants. I think that Rahner is trying to affirm that the Church is the unique means of salvation for all men. That is true. But it is so because God wanted it to be that way; God is perfectly capable of saving mankind without the mediation of the Church, and even without the Incarnation.)

and if this church of the beginning objectifies itself in scriptural documents at least in fact, and also does so necessarily given the historical and cultural presuppositions in which the church came to be, then in all of this together we have a point of departure for understand the essences of scripture.

The Church, which has its origins in Christ and the Apostles, commits to writing what is essential about itself (“objectifies itself”), a task that was rendered necessary by the historical and cultural situation of the early Church. (At least implicit in this assertion is that something of the historical situation of the early Church remains in the text of the Scriptures that we have today.)

By keeping these things in mind, it is much easier to understand what the Bible actually is.

It is also a point of departure from whose perspective we can arrive at an adequate and at the same time a critical understanding of what is really meant by the inspiration of scripture and by a binding canon of scripture.

From the same starting point, we can also understand what it means for the Bible to be “inspired” and what we mean by a “canon” of Scripture.

Since scripture is something derivative, it must be understood from the essential nature of the church, which is the escatological and irreversible permanence of Jesus Christ in history. It is to be understood from the perspective as something normative in the church.

The Bible is derived from the Church, not the other way around. The Church is (as we noted above) an indelible sign of both the Incarnation and the glory to come. The Church is, therefore, normative for us today (i.e., both in terms of what we believe and how we are to behave).