This abbreviation means “Application Programming Interface”. This is nothing but a special code for developing applications that will be accessible and understandable to every programmer. It consists of different functions and constants, formats and search requests.
The most popular API interface at the moment is the Google service. It includes about fifty different API modules, for example, for creating Android applications, API variations for working with AJAX, etc. It is not visible to ordinary users, since it is not necessary. Most often, the API is used by a script from a third-party website, which sends GET/POST requests, receives the result, and uses the necessary data as needed.