Representational State Transfer or REST is a set of design patterns that define how different applications interact and share information with each other (almost always over the Internet).
REST is designed to work with HTTP (hypertext transfer protocol). With HTTP, each “resource” on the Internet has its own URL. Resources can be individual products, clients, users, images, Web pages, files, or other site content.
HTTP uses a URL to identify a resource and retrieve it for a user. REST recommends a standard URL structure or pattern that developers should use for resources. This common standard makes it easier for applications to exchange information. Applications and URLs that use REST patterns are said to be RESTful.
The WordPress REST API is a plugin that allows developers to retrieve their WordPress sites’ content, users, taxonomies, and more in JSON format, and update them over HTTP. A programmer can use the API to pull their data into other applications and work with it in new ways. For example, the API allows you to use a WordPress site as the backend or database for a mobile app.