Quantum superposition is the phenomenon in which a quantum system can be described as a combination of multiple possible states before measurement.
For a particle in a quantum dot, the wavefunction can combine different allowed locations or energy states. The coefficients in that combination determine the probabilities of outcomes, while relative phases determine whether alternatives interfere constructively or destructively.
Superposition does not mean that a classical object is visibly split into separate copies. It is a mathematical description that produces experimentally testable interference effects. Measurement yields one result according to quantum probabilities.
Decoherence can destroy the observable interference when a system interacts with its environment. Superposition is also different from entanglement: entanglement concerns correlations between multiple systems, whereas superposition can describe a single system as well.