localhost is your private internet.
Before anything goes online, it runs on your own machine. Only you can see it, so you can break it safely.
01
Start
npm run dev
02
Open
localhost:3000
03
Change
edit and save
04
Refresh
see it update
05
Stop
Ctrl + C
The :3000 bit is a port, like a door number on your computer. Each running project gets its own door.
Local first. Online only when it's ready.