Convolution multiplies one sound by another in the frequency domain: every sample of the source gets replaced by a scaled, delayed copy of the whole impulse response (IR), then summed. Convolve with a room's IR and your sound is in that room; convolve with a decaying-noise burst and you've built a reverb; convolve with a comb of impulses and it rings at a pitch. The IRs here are generated in code — no files fetched — which is the whole point: a reverb is just decaying noise, an echo is a few taps. Or drop your own IR (or any sound) to convolve the two together.
Size/decay shape the noise tails; comb tone sets the pitch of the metallic comb. Custom uses a dropped file as the IR — that's cross-convolution.
play with your keyboard (a s d f …) — the source is convolved with the IR