What do these square brackets, triangles, and the numbers next to them mean, in Le Marteau sans maître by Pierre Boulez.
Lazy's answer is almost correct. The 2 and 3 at the beginning of each measure indicates the pattern to be conducted. The geometric figure indicates the duration of each beat. The square U indicates a quarter note (two eighths); the triangle indicates a dotted quarter (three eighths), and the single vertical stroke indicates one eighth.
See also my recent answer to the closely related question Is there a standard way to conduct 5/4 and 7/4, amongst other irregular time signatures?
These are conducting marks. A lines means one beat, a |_| shape means two beats, a triangle means three beats, a |_|_|_| four beats. In this case this does not refer to the actual beats of the measure, but to an 1/8 note subbeam structure.
So most likely the conductor is supposed to conduct one two beat gesture and one three beat gesture for the first measure, three two beat gestures for the second one and so on. The number indicate how many such subbeat divisions are within the current measure.