A comb filter is distortion produced by combining an electrical or acoustical signal with a delayed replica of itself. The result is constructive and destructive interference that results in peaks and nulls being introduced into the frequency response. Varying the time delay makes the comb filter sweep through its frequency range, picking out different harmonics as it moves. This is how a flanger works. When plotted to a linear frequency scale, the response resembles a comb, hence the name.
A less severe form of comb filtering occurs when the outputs from two microphones set up at different distances from a sound source are combined – a situation familiar to anyone who has mic'd up a drum kit, for example. Because the more distant mic receives less level than the close mic, the depth of the filtering isn't as pronounced as in our flanger example, but it can still compromise the overall sound.