Superposition is one of the defining features of quantum mechanics. A quantum system, such as a qubit, can exist in a combination of its possible states at the same time. A classical bit must be either 0 or 1, but a qubit in superposition holds a controlled blend of both, described by a quantum state that assigns a probability to each outcome.
When the system is measured, the superposition ends. The qubit settles into one of its possible states, chosen at random according to the probabilities the quantum state encodes. Before that measurement, the qubit genuinely occupies both possibilities, and this is what allows a quantum computer to explore many computational paths.
Superposition works together with entanglement to give quantum computers their power. For a silicon spin qubit, superposition corresponds to the electron’s spin pointing not simply up or down but in a controlled combination of the two. Preserving that superposition against decoherence is central to reliable computation.