A terminal emulation that enables a user to connect to a remote host or device using a telnet client, usually over port 23. For example, typing telnet hostname would connect a user to a hostname named hostname. Telnet enables a user to manage an account or device remotely. For example, a user may telnet into a computer that hosts their website to manage their files remotely. In the image is an example of a telnet session. As shown, a telnet session is a command line interface.
Telnet is considered insecure because it transfers all data in clear text. Which means if a user was sniffing a network, it's very possible they could grab your username and password as they were being transmitted. Users who are concerned about the data being transmitted should consider SSH instead of telnet.