To chop a sample is to divide it into smaller pieces and then perform or program those pieces as if they were the notes of an instrument. Modern samplers can place the cuts automatically at each transient, split a loop into equal musical divisions, or let the producer tap in slice points by hand while the audio plays. On an Akai MPC the slices can then be converted into a drum program spread across the pads.
Chopping is how a phrase from an old jazz record stops being a quotation and becomes a new idea. A producer might keep two chords from an intro, a single vibraphone note and a cymbal swell, then replay them in a different order over fresh drums. Short, rearranged chops are a hallmark of jazz lo-fi and of the hip hop tradition it grew out of.