The short version is this: on the tinest levels of reality, stuff behaves differently. It's neither here nor there, neither point nor wave (like water). But up here on the big level where we live, everything is way more specific. Stuff is just where it should be and it's normal, everyday stuff.
So where is the dividing line? At what point does the fuzziness of the quantum level turn into the hard-as-rock reality we know? This experiment involves large molecules, not just single photons. Yet they acted just like quantum particles in the famous double-slit experiment. This effectively moved the dividing line up toward our big level, just a bit. This is shocking. Trust me on this. So where the heck is this dividing line? It's one of the most interesting questions in science.