To begin explaining comb filtering we first have to explain something called destructive phase interference. Simply put, destructive phasing happens when a waveform is presented to a second identical version of itself that has the exact same size but is perfectly out of phase (180 degrees), meaning that the waves peaks and dips cycle in opposition to each other. The result is near complete cancellation of the two wave forms. The two waveforms add together to equal almost nothing. Weird, right? Physics is crazy. 

On the other hand, if two identical signals interact with each other that are perfectly in phase, the two signals sum together and the sound is perceived to be 6 dB louder. So when two identical signals interact, depending on their mutual phase relationship, we can end up with either a louder signal, or almost no signal at all. These are the two extremes of this phenomenon. It is possible to end up with varying degrees of this as two signals might be within different phase ranges. And this is where we start to experience comb filtering.