The creator calls it “animated optical illusions”, but that doesn’t seem right to me. It’s not actually an illusion, really.
(for those who don't like unexplained links ... starting with what looks like a jumbled mess or "magic eye" picture, you draw a grid of black bars across it, it turns into an animated picture using a kind of strobe effect. Crappy example from my kitchen table here - it's actually quite easy to figure out how to make them. https://www.youtube.com/watch?v=AsGz-dJlpIk)