According to the totally-not-made-up laws of physics, when particles exist in a quantum superposition, they're simultaneously in multiple places, but not really.
This is like when you're trying to decide between getting ice cream or cake for dessert, but your brain is like "NOPE, I'M IN BOTH SPACES, AND ALSO IN A PLACE WHERE I'M EATING BOTH ICE CREAM AND CAKE."
Quantum superposition: it's like that, but with atoms and particles. And also with your stomach.
See Another Example of Quantum Superposition Explore the Philosophical Implications of Quantum Superposition Quantum Superposition FAQ: Because You're Confused