Mathematically, it refers to a property of solutions to the Schrödinger equation; since the Schrödinger equation is linear, any linear combination of solutions will also be a solution. Quantum superposition is a fundamental principle of quantum mechanics.
Definition:
It states that, much like waves in classical physics, any two (or more) quantum states can be added together (“superposed”) and the result will be another valid quantum state; and conversely, that every quantum state can be represented as a sum of two or more other distinct states.
Source:
Wikipedia Contributors. “Quantum Superposition.” Wikipedia, Wikimedia Foundation, 8 Nov. 2020, en.wikipedia.org/wiki/Quantum_superposition. Accessed 10 Nov. 2020.
In 1935, Erwin Schrödinger devised a well-familiar thought experiment, now familiar as Schrödinger’s cat, which highlighted this dissonance between quantum mechanics and classical physics.
It is natural to ask why ordinary everyday objects and events do not seem to display quantum mechanical features such as superposition.
One modern view is this mystery is explained by quantum decoherence.
Indeed, this is sometimes regarded as “mysterious”, in support of instance by Richard Feynman.
Source:
[1] Wikipedia Contributors. “Quantum Superposition.” Wikipedia, Wikimedia Foundation, 8 Nov. 2020, en.wikipedia.org/wiki/Quantum_superposition. Accessed 10 Nov. 2020.