Zero-crossing edits: cutting or splicing audio where the waveform crosses the zero line (0 amplitude). At this point, there is no abrupt voltage change, so the edit is silent. Cutting at a non-zero crossing creates a sudden jump in voltage — which the speaker reproduces as a click or pop.
Most DAWs have a 'snap to zero crossing' option that automatically finds the nearest zero crossing when you make an edit. For loop creation, both the start and end points must be at zero crossings to create a seamless loop. Crossfades (overlapping the end of one clip with the start of the next) are an alternative that works at any edit point.