Superposition is the idea that a quantum system can hold two or more states simultaneously rather than being in just one. An electron can be in a superposition of spin up and spin down, and a photon can be in a superposition of two different paths through an experiment.
This is not the same as simply not knowing which state it is in. The system genuinely occupies both possibilities at once, and interference experiments prove it, since the two states can combine and cancel each other out in ways that only make sense if both were really present.
Superposition collapses to a single definite outcome the moment it is measured. It is the foundation of quantum computing, where a qubit in superposition can represent both 0 and 1 at the same time, letting certain calculations explore many possibilities in parallel.