Localhost
Your computer pretending to be a website — only you can see it.
What is Localhost?
When you build an app and run it locally, your browser shows something like localhost:3000. That's your app running on your own computer. Nobody else on the internet can see it yet. It's your private testing space before you deploy it for real.
Think of it like trying on clothes in a fitting room before walking out in public. Localhost lets you test and tweak your app privately.
In Vibe Coding
When you run npm run dev in Cursor, your app starts at localhost:3000. Lovable and Bolt show you a preview instead, but it's the same idea — a private version of your app that only you can see.
Example
For example: You're building a landing page in Cursor. You run npm run dev, open localhost:3000 in Chrome, and see your page. You make a change, save the file, and the page updates instantly.