The Schrödinger Equation

A wavefunction is a mathematical function that describes the quantum state of a system in terms of the probability of finding the system in a particular state. Or, you know, just a bunch of squiggly lines.

But wait, what about the wave-particle duality? That's a whole other can of worms.

And don't even get me started on the Heisenberg Uncertainty Principle... that's just a wild ride.