Backend
aka: server
The Quick Version
The part that does work you don't want happening in someone's browser.
How it actually works
The backend is code that runs on a computer you control (a server), not on the visitor's device. It handles things that must be trusted or private: charging cards, sending email, reading and writing the database, checking passwords.
Why does it matter?
Anything a visitor should not be able to tamper with belongs on the backend.
Example
Checking that a password is correct happens on the backend.