An API (Application Programming Interface) is a set of definitions and protocols that facilitate the creation and integration of software applications by allowing them to query and exchange with computer systems. The client queries the API to access information rather than the source system directly.
The client can be an application (website, mobile application) as well as a developer who directly queries the API. The source system can be a set of databases, an operational database, etc. The API itself is composed of a set of libraries and functions that allow to receive calls and data from clients and to make the link with the source systems. APIs have the advantage of simplifying operations and increasing security.