An engine swap is the process of removing a car

Automobile

An automobile, autocar, motor car or car is a wheeled motor vehicle used for transporting passengers, which also carries its own engine or motor...

's engine

Internal combustion engine

The internal combustion engine is an engine in which the combustion of a fuel occurs with an oxidizer in a combustion chamber. In an internal combustion engine, the expansion of the high-temperature and high -pressure gases produced by combustion apply direct force to some component of the engine...

 and replacing it with another. This is done either because of failure, or to install a different engine, usually one that is more powerful or more modern and maintainable.

An engine swap can either be to another engine intended to work in the car by the manufacturer, or one totally different. The former is much simpler than the latter.