Zero Crossing and DC Offset

When audio is digitized, there is a portion above the zero axis and a portion below. Where the signal crosses the axis, there's no voltage and the signal is absolutely silent at that spot. This location is called a "zero crossing". Ideally, all edits would be made at zero crossings so that no pop or noise surge would be produced.
But since, on most DAWs, this generally doesn't happen, the software usually creates a small fade to eliminate any pop or noise.
With analog tape, engineers use an Editall splicing block that creates a fade by the edit angle. Also, film editor cutting magnetic film with a guillotine splicer, will usually scrape the oxide at the edit point with a razor blade to create a fade-in and eliminate any pop.
With digital audio, the average of the amplitudes (both above and below the zero axis) should theoretically be zero, with just as much signal above the horizontal axis as below.
However, in some cases that doesn't happen, and the signal is offset from the zero axis. This situation is known as DC bias, DC coefficient, or more commonly DC offset. It's caused by technical issues when the files were recorded, and even simply playing the file will produce a noticeable pop.
To correct for this, many DAWs have a DC Offset plug-in, which removes the problem so that the mean amplitude returns to zero.