Zero crossing
In audio editing (and electronics in general), a zero crossing is when the signal of a waveform crosses from a positive value to a negative value, or vice versa – that is, the signal temporarily has a value of zero.
The purple lines denote zero crossings.
When editing audio, if the beginning or end of an audio clip is not at a zero crossing, there will be an audible “pop” or “click” sound. If the clip does indeed end at a zero crossing, but a pop sound is still audible, it may be that the slope of the waveform at that point is too steep.
An easier editing method is to use a very brief fade-in/fade-out.