Young's double slit experiment is one of the things that started the field of quantum mechanics. Basically, you shine a single light (or beam of particles) at two slits at the same time. Behind the slits is a screen. You’d expect to see two shards of light hit the screen, but in fact what you actually get is an interference pattern, like you would get if you sent a wave at the two slits. This was the start of understanding light as both a wave and a particle (a waveicle).
When you read into it more, it starts getting very weird indeed. For example, if you look at the light before it passes through the slits (count the particles or whatever), the interference pattern disappears instantly. Ok you might say. Fair enough. When you look at it, it decides it’s a particle and not a wave after all. However, if you look at it on the other side of the slits - after it has done its thing, you still loose the pattern. This means it ‘knows’ you are going to measure it before it even travels through the slits.
Here are some cool examples of interference patterns: