The word breaks down simply: “host” is the machine serving the app, “local” means it’s your own. Localhost is that machine running a site privately, before anyone else can reach it.
Think about setting up a trade show booth in your garage before the event. You arrange the table, test the lights, and fix what looks wrong, and nobody at the show sees the messy version. Localhost is the garage. A developer or agent runs the site locally, opens an address like localhost:3000, and tests before deployment.
How it shows up
Ask Claude Code or Codex to build a small app and it may start a dev server that prints a URL like http://localhost:4321. Open it and you see the app running from your machine. This is why localhost is safer for drafts: the AI can change the layout, break a button, fix it, and reload without touching the public site. It’s also why a localhost link doesn’t work for anyone else. Send a client http://localhost:3000 and their computer looks at itself, not yours, like directions to “my garage” while they stand at their own house. It often pairs with a sandbox, where experiments happen without touching production data.
Why you care
Localhost separates “we’re testing” from “we shipped.” The app runs privately, you test it, then you deploy it to a real server others can reach. If an agent says the site is running locally, the world can’t see it yet. It also gives a better review loop: instead of judging a screenshot, you open the local app, click the actual buttons, and say what feels wrong.