REST API is a style of API that treats data as resources with URLs. You use ordinary HTTP methods: GET to read, POST to create, PUT or PATCH to update, DELETE to remove. A product might live at a stable address. The response is often JSON.
It is not GraphQL. GraphQL lets the client ask for a custom set of fields in one query. REST uses the resource URLs the server published. A webhook runs the other direction: the other system calls you when something happens, instead of you requesting it. The backend usually exposes the REST API. A headless CMS or a microservice often speaks it. Rate limiting keeps one client from hammering the door.
A real-life example of a REST API
A Decatur shop’s warehouse tracks stock in one system. The online catalog needs that number on the site. A REST read of the product keeps the page from selling a vase that already left the shelf. When an order is paid, a write updates the count. Nobody retypes it at night.
Ask for the resource, change the resource, and handle the error when it is gone. Primer: What Is an API? and get ready to REST.