In computer networking, localhost (meaning “this computer”) is the standard hostname given to the address of the loopback network interface.
Localhost always translates to the loopback IP address 127.0.0.1 in IPv4.
It is also used instead of the hostname of a computer. For example, directing a web browser installed on a system running an HTTP server to http://localhost will display the home page of the local web site.
Source: Wikipedia – Localhost.
The :80 part is the TCP port. You can consider these ports as communications endpoints on a particular IP address (in the case of localhost – 127.0.0.1). The IANA is responsible for maintaining the official assignments of standard port numbers for specific services. Port 80 happens to be the standard port for HTTP.