developmentbeginner
Port
The number after localhost that tells your computer which app to show.
What is Port?
Your computer can run many apps at once. Ports separate them. localhost:3000 is one app, localhost:8080 is another. Think of it like apartment numbers — same building, different units.
If you see an error saying "port already in use," it means another app is already running on that port. Close it or pick a different port.
In Vibe Coding
In Cursor, your frontend usually runs on port 3000 and your backend might run on port 8080. If you see "port already in use," close the other app or change the port in your config.
Example
For example: Your Next.js app runs on localhost:3000 and your Supabase local dashboard runs on localhost:54323. Different ports, same computer.