"Localhost" is the default hostname that is assigned to the loopback network interface of a computer. In simple words, it's a way to refer to your own computer. When talking about "opening localhost," it means accessing a web server that is running on own machine.

http:// localhost

Localhost is commonly used for web development and testing. It allows you to work on websites or web applications locally before deploying them to a live server.

Can't open localhost try this steps::

1. Start a Local Server: You need to have a web server running on your computer. For example, tools like XAMPP, MAMP, or simply Pythong's build-in HTTP server.
2. Access Localhost: Once your server is running, open a web browser and type `http://localhost` in browser's address bar and Press Enter.