Localhost
aka: dev serveraka: 127.0.0.1
The Quick Version
Your website running on your own computer, visible only to you.
How it actually works
localhost is your own machine. A dev server serves your project there on a port (like 5173) with hot reloading so edits appear instantly.
Why does it matter?
It is where you test everything before the internet gets a vote.
Example
http://localhost:5173