The software world
localhost
127.0.0.1
A name that always means this computer. When Claude starts a project and tells you to open
http://localhost:3000, the site is running as a process on your own machine — not on the internet, and not visible to anyone else. Why it matters
Part of Software words you'll hear → It's the word that separates "it works" from "it's live", and mixing them up is the most common source of a false sense of progress. A site on localhost exists only while that process is running and only for you; closing the terminal or the app stops it. Making it reachable by other people is a separate step called deploying. The number after the colon is the port — which is why port 3000 already in use means something else is already listening there.