A comb filter changes the frequency response in a specific way that looks sort of like teeth on a hair comb:

In this graph, ignore the falling HF which is due to the Radio Shack SPL meter's response.

This kind of response occurs when a signal is combined with a delayed version of itself. The lowest null occurs when the delay time equals 1/2 wavelength (180 degrees out of phase), and subsequent peaks and nulls occur at regular intervals of 2/2, 3/2, 4/2 etc wavelengths.

More here:

http://www.realtraps.com/art_spaces.htm

and here:

http://www.realtraps.com/video_comb.htm

--Ethan

If you read the article I linked (the video is admittedly pretty boring), see the sidebar at the end that talks about comb filtering in AM and FM radio, and microwave ovens. Comb filtering is very prevalent all over the place!

--Ethan