Superposition is the quantum property that allows a system to combine multiple possible states before measurement.
A quantum state can be written as a linear combination of other allowed states. For example, an electron can be represented as a combination of spin-up and spin-down states along a chosen measurement axis. The coefficients in that combination determine the probabilities of the possible measurement results.
Superposition does not mean that every classical description is literally occurring in an ordinary macroscopic sense. Interaction with the environment can cause decoherence, making interference between alternatives unobservable. The famous double-slit experiment demonstrates the principle because individual particles build an interference pattern when path information is unavailable.