In more detail
In a REST API, GET /users/42 reads a user, POST /users creates one, PUT /users/42 replaces it and DELETE /users/42 removes it, with the result signalled by HTTP status codes. Resources are nouns in the URL (/users/42/orders) and the method is the verb; PATCH changes part of a resource. GET, PUT and DELETE are idempotent, meaning that repeating them has the same effect as sending them once; POST is not. Each request carries everything needed to process it, so servers stay stateless and are easy to scale. Bodies are usually JSON.
Why it matters
Idempotency decides what is safe to retry: repeating a PUT after a timeout is harmless, while repeating a POST may create a second order. Many APIs called REST bend the rules, so the documentation, not the label, tells you how each endpoint behaves.
Example
POST /users HTTP/1.1
Content-Type: application/json
{"name": "Ada"}
HTTP/1.1 201 Created
Location: /users/43
201 Created and the Location header tell the client where the new resource lives. See HTTP Status Codes Cheat Sheet.
Common confusion
An API is any defined interface between programs; REST is one design style for web APIs, alongside GraphQL and gRPC.