From my experience as a beginner in web development, ‘localhost:3000’ is basically the address your computer uses to run a web server locally. When I started using React, the development server would automatically run on port 3000, so visiting ‘localhost:3000’ in my browser showed me my app in real-time.
It’s super handy because you don’t need an internet connection,your computer is both the server and the client. And, to be honest, it took me a while to realize how much easier this made debugging and testing my work in isolation.