Telnet is an older Internet utility that lets you log on to remote computer systems.
A Telnet program gives you a character-based terminal window on another system. You get a login prompt on that system. If you have permitted access, then you can work on that system as you would if you were sitting next to it.
Generally, Telnet has been used by people who have logins on remote systems and want to do serious work there.
Telnet has also some additional uses that are more relevant to people who are exploring the Internet. Most notably, you can use Telnet to connect to thousands of catalogs at libraries around the world. This capability is wonderful for anyone doing serious research. Imagine being able to find out which books in your particular discipline are available at a number of speciality libraries in remote locations, all while you plug away at your desk.