A comb filter mixes a signal with a short delayed copy of itself. The delay causes some frequencies to reinforce and others to cancel, producing evenly spaced peaks and notches that look like the teeth of a comb. Changing the delay time shifts the pattern, which makes flanging and chorus-like sweeps.
With feedback and a very short delay, a comb filter can ring at a pitch, which is the basis of Karplus-Strong plucked strings.