Frontend
aka: clientaka: ui
The Quick Version
The part people actually see and click.
How it actually works
The frontend is the code that runs inside a visitor's browser: the HTML structure, the CSS styling and the JavaScript that reacts when someone types or clicks. It is downloaded to their device and executed there.
Why does it matter?
Anything you can see or touch on your site is frontend. When you say 'make it look better', you are talking about the frontend.
Example
The buttons, navigation and layout of this page are frontend.