API

API (Application Programming Interface) is a set of routines, data structures, protocols, and tools used for creation of software applications. API may be considered as a list of commands sent by one program to another to use the functions of each other.

In web development API allows interaction of programs running on the servers that you connect to from your browser. Websites have APIs as well, so it helps to limit outside program access to a specific set of features. That saves time for server requests and means you can use resources only available on the server (for instance, logins and swords, etc.).