The Double-Slit Experiment is one of the most important in quantum physics. It tells us that there is something inherently different about the fundamental laws of nature compared with our everyday experience. Richard Feynman called it "the heart of quantum mechanics. In reality, it contains [its] only mystery."

The essential idea is that particles travel as delocalized waves that can pass through both slits at once, but hit the screen as discrete particles. This leads to an interference pattern that builds up on the screen, particle by particle. This interference pattern can be destroyed by putting detectors on the slits to see which one it actually goes through.

An entire book is dedicated to this one demonstration: Through Two Doors at Once: The Elegant Experiment That Captures the Enigma of Our Quantum Reality Here is a review.

[NOTE: The middle section doesn't quite line up correctly with the other two.]