What is useless/meaningless speech?

It's detailed in M. 114 which is translated as follows (see page 147, i.e. page 11/31 of the PDF):

(10) SPEAKING FRIVOLOUSLY. Furthermore, he chatters frivolously [utters useless talk]—
he speaks at the wrong time, what is false, what is not connected with the goal [unbeneficial], what is not the teaching [Dharma], what is not the discipline [Vinaya];
at the wrong time, he speaks words not worth treasuring, baseless, undefined [rambling], not connected with the goal.

... and page 148 (i.e. page 12/31 of the PDF):

(14) NOT SPEAKING FRIVOLOUSLY. Having given up frivolous chatter, he refrains from frivolous chatter—
he speaks at the right time, what is true, what is connected with the goal [beneficial], what is the teaching, what is the discipline;
he speaks words worth treasuring, at the right time, well-founded, well-defined [not rambling], connected with the goal

Various copies of the definition of the Kamma Patha (for example here) say,

Both lists occur repeatedly, e.g. in A.X.28, 176; M.9; they are explained in detail in M.114, and in Com. to M.9 (R. Und., p. 14), Atthasālini Tr. I, 126ff.

I copied the "explanation in detail from M 114" above, you might find further commentary in the others of these references.

In addition found in Sevitabbâsevitabba Sutta there are 32 types of chatter which is called animal chatter which falls under the category of idle / frivolous chatter:

1. Rajakatha – Talk about kings
2. Corakatha – Talk about robbers
3. Mahamatta katha – Talk about ministers of state
4. Senakatha – Talk about armies
5. Bhayakatha – Talk about dangers
6. Yuddhakatha – Talk about battles
7. Annakatha – Talk about food
8. Panakatha – Talk about drinks
9. Vatthakatha – Talk about clothing
10. Sayanakatha – Talk about dwellings
11. Malakatha – Talk about garlands
12. Gandhakatha – Talk about perfumes
13. Natikatha – Talk about relations
14. Yanakatha – Talk about vehicles
15. Gamakatha – Talk about villages
16. Nigamakatha -Talk about market towns
17. Nagarakatha – Talk about towns
18. Janapadakatha – Talk about districts
19. Itthikatha – Talk about women/men
20. Surakatha – Talk about heroes
21. Visakhakatha – Talk about streets
22. Kumbhatthanakatha – Talk about watering places
23. Pubbapeta-katha – Talk about relatives who have passed away
24. Nanattakatha – Tittle-tattle
25. Lokakkhayika katha – talk about the origin of the world
26. Samuddakkhayikakatha – Talk about the origin of the ocean
27. Itibhavabhava katha – Talk about Eternity belief
28. Itibhavabhava katha – Talk about annihilation belief
29. Itibhavabhava katha – Talk about worldly gain
30. Itibhavabhava katha – Talk about worldly loss
31. Itibhavabhava katha – Talk about self-indulgence
32. Itibhavabhava katha – Talk about self-mortification

Sourced from: Tiracchana-Katha Unskilful Talk which is an adaptation from Venerable Ledi Sayadaw’s Magganga-Dipani: The Manual of The Constituents of the Noble Path.

Instead of memorizing specific type of talk you can discern what it idle chatter through the fundamentals, i.e.,:

1. is not beneficial,
2. not relevant to the fundamentals of the holy life,
3. does not leads to revulsion,
4. does not leads to dispassion,
5. does not leads to cessation,
6. does not leads to peace,
7. does not leads to direct knowledge,
8. does not leads to enlightenment,
9. does not leads to Nibbana

Adopted from: Tiracchana-Katha Unskilful Talk

... or in general context ...

1. he speaks at the wrong time,
2. what is false,
3. what is not connected with the goal [unbeneficial],
4. what is not the teaching [Dharma]¹,
5. what is not the discipline [Vinaya];
6. he speaks words not worth treasuring,
7. baseless,
8. undefined [rambling],
9. not connected with the goal

... or more succinctly, chatter in which, unwholesome states increase and wholesome states decrease.

See: Sevitabbâsevitabba Sutta and answer by @ChrisW

Also talk you should engage in:

1. Talk about wanting little,
2. about contentment,
3. seclusion,
4. solitude,
5. energetic striving,
6. virtue,
7. concentration,
8. insight,
9. release,
10. release by knowing and seeing

In addition the Buddha mentions you should talk about the 4 Noble Truths as this lead to:

1. is beneficial,
2. relevant to the fundamentals of the holy life,
3. leads to revulsion,
4. leads to dispassion,
5. leads to cessation,
6. leads to peace,
7. leads to direct knowledge,
8. leads to enlightenment,
9. leads to Nibbana

Adopted from: Tiracchana-Katha Unskilful Talk

... or in general context ...

1. he speaks at the right time,
2. what is true,
3. what is connected with the goal [beneficial],
4. what is the teaching¹,
5. what is the discipline;
6. he speaks words worth treasuring,
7. well-founded,
8. well-defined [not rambling],
9. connected with the goal

... or more succinctly, chatter in which, unwholesome states decrease and wholesome states increase.

See: Sevitabbâsevitabba Sutta and answer by @ChrisW

Several suttas contain this list but all of them list only 27. However, as usual, the Commentators obsessed with splitting hairs have stretched it to 32 by splitting the last one into 6.

The name "Detis Katha" is also theirs. The Suttas called the original ones "Tirracchana katha" which literally means ‘animal talk’.

Incidentally this list is not for us to learn and remember. These topics are for avoiding. Several suttas say that we should instead be discussing only the Noble Truth.

Source: Dethis Katha (32 types of unskillful talks) by user Gayan

Furthermore the blog Tiracchana-Katha Unskilful Talk has comprehensive treatment of this topic.

¹

Regarding skillful and unskillful chattering regarding the Dhamma there is 8:

1. This Dhamma is for reducing desire, not for increasing desire.
2. This Dhamma is for increasing contentment, not for increasing discontentment.
3. This Dhamma is for cultivating seclusion, not for becoming sociable.
4. This Dhamma is for becoming energetic, not for becoming lazy.
5. This Dhamma is for developing mindfulness, not developing unmindfulness.
6. This Dhamma is for developing composure, not for making restlessness.
7. This Dhamma is for increasing wisdom, not for decreasing wisdom.
8. This Dhamma is for delighting in freedom from impediments, not for delighting in impediment.

Sourced from: Eight Great Noble Thoughts and Papanca [proliferation] and Nippapanca [non-proliferation]