You can think of an API as the "contract" that allows two pieces of software to interact with each other. When you want your application to interact with another one, you need to know what data it requires and what data it offers. These requirements are specified in the form of an interface description or contract, which is part of the API.
APIs are used by developers all over the world to create apps for websites, mobile devices and more.
Every time you send a Facebook message, you're using an API.