Young's double slit experiment.

When which slit a photon goes through is unobserved, it behaves like a wave and self interferes so many photons create an interference pattern with stripes where self-interactions prevented any photons from appearing.

When the photon is interacted with in a way which leaves permanent information about which slit it went through, it behaves like a particle and the pattern from many photons looks 'ballistic' like you were shooting tiny balls through each slit.

So in the meme when he's not looking at the slits, there's stripes, and when he's looking it's a ballistic pattern.