Backend or back-end or BE is a back-facing layer or application. It's an application that will respond to client (or frontend) requests.
It can be:
- An application that runs on a server that checks whether username and password are matching or not
- An cloud-based application that queries news feed
Let's think about a restaurant. They have a front-facing restaurant and a kitchen. The kitchen is the backend. Guests will order menus and chefs will cook those menus. Just like how a backend will serve client requests.
Most of the time a backend will have API that lets the client make a request.