Application Performance Interface is knwon as API. It is used to transfer data between websites and applications

More technically,

It is a developer-oriented feature of WordPress. It provides data access to the content of your site, and implements the same authentication restrictions — content that is public on your site is generally publicly accessible via the REST API, while private content, password-protected content, internal users, custom post types, and metadata is only available with authentication or if you specifically set it to be so.

How API Works?

The WordPress REST API provides an interface for applications to interact with your WordPress site by sending and receiving data as JSON (JavaScript Object Notation) objects.

What is Rest API?

representational state transfer is programming interface that conforms to the constraints of REST architectural style and allows for interaction with RESTful web services. REST stands for representational state transfer in API.