Telnet was and is the way of connecting to computers on the Internet. Before the World Wide Web made graphical access to the Internet possible, computers on the Internet understood only typed commands very much like DOS.
Telnet is designed to allow a user to log in to a foreign machine and execute commands there. Telnet works as though you are at the console of the remote machine, as if you physically approached the remote machine, tuned it on, and began working on it. Now that the World Wide Web has become the preferred way to access most resources, Telnet is seldom used, except for special applications, system administration, and to access archaic systems.