A good starting point is the REST API Handbook at WordPress.org, https://developer.wordpress.org/rest-api/.
The handbook does a good job of explaining what a REST API is and it’s benefits in the WordPress context. The key point to appreciate is that we’re talking here about the WP site itself being a “provider of an API”. This is a unique and special feature of WordPress, something not found in most systems. In nearly any framework or CMS you can integrate API’s, but having the system itself “become an API”? That’s a next-level feature.
A good starting point for seeing how the API data looks is to visit any WP site and navigate to /wp-json/. This base path will show you all the available routes (endpoints) as well as some other data about the site.