As stated in the WordPress handbook:

The WordPress REST API provides API endpoints for WordPress data types that allow developers to interact with sites remotely by sending and receiving JSON (JavaScript Object Notation) objects. JSON is an open standard data format that is lightweight and human-readable, and looks like Objects do in JavaScript, hence the name. When you send content or make a request to the API, the response will be returned in JSON. This enables developers to create, read and update WordPress content from client-side JavaScript or from external applications (even those written in languages beyond PHP).

Useful links

Building custom REST API routes using object-oriented php