Quantum objects, unlike regular, familiar objects, have the counterintuitive ability to be in multiple states at once. If you flip a normal coin, you know it will land either heads-up or tails-up. But if you flip a quantum coin, and immediately cover it up without checking its status, it will exist in a "superposition" of the heads-up and tails-up positions, a combination of the two states. Until you observe the coin, you can't know its true state, but you can know its "wavefunction," which describes the probability that it's in any given state. Once you peek at the coin, its wavefunction collapses into just one of the possible states, either heads or tails. The most famous example of this phenomenon is "Schrödinger's cat".