Lay two window screens over each other and giant dark ripples appear that exist in neither screen. That is moiré: when fine regular patterns overlap slightly misaligned, their difference becomes visible as a much larger pattern that swims and lurches with the tiniest relative movement. The magnification is the striking part, a half-degree rotation of one grid sweeps the interference bands across the whole field.
As artifact, moiré plagues cameras (fine fabric against sensor grids), screens photographed by other screens, and any resampling of periodic detail, which is why dithering and anti-alias filters exist. As instrument, op-art painters like Bridget Riley built careers on it, and analog VJs discovered early that two layered gratings turn a projector into an interference synthesizer.
The reactive appeal is sensitivity: because tiny movements produce huge visible change, moiré converts small parameter motion into full-screen activity almost free. Two line fields with audio-driven rotation and scale produce breathing, rippling geometry with nearly no computation. The same sensitivity makes it dangerous, since it aliases brutally when rendered carelessly; the good versions supersample or band-limit the gratings so the interference is the only artifact on screen.