One place for a particular request
An API lets one piece of software ask another for information or an action. For a web API, an endpoint pairs an address with an action. For example, a shop app might use GET /orders/42 to read order 42 or POST /orders to create an order.
GET and POST are HTTP methods that tell the server what action is requested. The part after them points to the relevant order or orders. The server then responds with the result or an error, often marked by a status code.
Knowing the address does not grant access
An API may require an access key, limit which orders an app can see, or cap how often it can send requests. Its documentation explains what to send, what comes back, and how errors are reported.