Question

what is superposition?​

Answer 1

Superposition is the action of placing one thing on or above another, especially so that they coincide.

Answer 2

The superposition principle is the idea that a system is in all possible states at the same time, until it is measured. After measurement it then falls to one of the basis states that form the superposition, thus destroying the original configuration. The superposition principle explains the "quantum weirdness" observed with many experiments. A classic example of this is the double-slit experiment. Here, two slits in a barrier allow for the passage of (for example) electrons. The result of this experiment is an interference pattern not predicted by classical mechanics.