This is a demonstration of comb filtering. When a sound is slightly delayed, then added back to itself, constructive and destructive interference occurs. This can be caused by a nearby reflective surface that the sound bounces off of. Comb filtering causes regularly spaced spikes and drops in the frequency response, making it look like a comb, which is where it gets its name.
Hopefully, seeing the frequency response, hearing the sound, and watching as I move the reflective surface closer to the sound source and mic makes it clear exactly what is happening here!