It's a phenomenon where two particles become connected, no matter the distance between them. Like when you order a latte at the coffee shop, but your coworker, who's sitting on the other side of the office, somehow knows exactly what you want.
Or, in this case, when you put your cup down for just a second, and someone in the back of the room, who's clearly not paying attention, knocks over their own cup, and they're now connected in a quantum dance of coffee-staining.
We've all been there, haven't we?
Quantum Entanglement in the Coffee Shop: A Guide