The Superposition theorem is a basic and Fundamental Law of Electrical Engineering. It can be used to solve the complex circuits which have multiple current/voltage sources connected with each other in a single circuit. While applying theorem, only one source is considered whereas the other sources are replaced, either by a short circuit or by an open circuit.
The current source is replaced with the open circuit.
The short source is replaced by the short circuit.
In this video the Superposition theorem is briefed with a solved example.