Backend
The backend is the part of an app that runs out of sight on a server: it saves data, checks who you are and keeps secrets.
In a simple example
When you press Book, the front screen sends your choice to the backend, which saves it and says OK.
Why it matters to you
Anything private, like keys or customer data, must live in the backend, never in the page people can inspect.
Do not confuse with
The backend is not a place people visit. It is the engine room behind the screen.