It’s the Young’s double slit experiment. It proves that light (or electrons, or even small bacteria) is both a particle and a wave.

There is a quirk of quantum mechanics. When you observe a system, you fundamentally change it. In scientific terms “observe” has a very different meaning to layman usage. This leads to a lot of woo around the topic. In practice, observing is measuring. In quantum mechanics, the measurement system is of the same scale as the system being measured.

Imagine observing a good train, by bouncing BB bullets off it with a gun. That is classical measurement. You can assume the BBs had no effect on the train.

Now imagine the same measurement. However you are measuring how a bunch of glass playing cards are balanced in a house of cards. You can tell a lot still, but the BBs will smash it up doing so. This is quantum measurements.

In the first, the observer is independent of the system. In the second, the observer is a fundamental part of the system, and so can change its way of functioning.