Errors as instruments
Glitch is the sound of digital things going wrong: a CD skipping, a buffer stuttering, a file read as audio. Since the late 1990s, electronic musicians have used those errors as material, and today glitches show up in IDM, dubstep, hyperpop and trap transitions, as well as in game interfaces and video edits.
What to look for
Short glitches, under half a second, work as rhythmic hits. Longer ones make good transitions or background texture. Listen for clicks at the end of a sample: a quick fade avoids an unintended pop, unless the pop is the point. Because many glitches are very bright, a gentle low-pass around 10 kHz can stop them from sounding harsh on headphones.
Most glitch samples on free sites are made from scratch by sound designers, so licences are often permissive. The verdict on each card confirms it.