"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.
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::
Here are some useful folders and ports on web servers
xampp, apache, tornado etc...