The backend receives requests, verifies permissions, reads or writes data, calls external services, and returns responses. It is where trusted logic belongs. A good backend protects the product even when the frontend or user input is messy.
ELI5
Think of Backend as a named building block in a big LEGO app. The server-side part of an app that handles APIs, databases, secrets, auth, and business logic.
In practice
Use it when you are mapping how the app is structured or explaining a feature to a teammate or agent. In practice, define the owner, input, output, and failure mode before you rely on it.