It’s just proof that light behaves as a wave, because it generates an interference pattern like the first picture. The second picture is how it would theoretically behave if it was (only) a particle, which it isn’t. The proof that light acts like a particle comes from a second experiment proposed by Einstein dealing with the photoelectric effect.

This article is meant for kids but it explains things pretty plainly. The duality is unexplained, but the experiment is well understood.