Build Basics / Standard term
Localhost
A hostname that resolves to the loopback interface of the machine making the request.
Opening http://localhost:3000 tells the browser to contact port 3000 on its own machine. Another computer's localhost points to that other computer, not yours. The underlying development server may also bind to a network interface, and tunnels or port forwarding can expose it elsewhere, so localhost use should not be treated as proof that the service or its data is private.
Builder example
Running on localhost lets you preview and test a project before anyone else sees it, which keeps half-finished work and broken screens off the public web. When you ask an AI assistant to build a feature, it usually runs the project locally first so you can confirm the password reset flow or the new form behaves before you spend any time deploying. Treat the localhost link as your private rehearsal space, then deploy once the behavior is right.
Common confusion: The hostname is local, but the application can still call remote APIs, use production credentials, or be exposed through another address. Localhost describes the connection target, not the safety of the environment.