Telnet
In plain terms
An old, unencrypted protocol for remote terminal access.
Going deeper
Telnet opens a text session to a remote machine but sends everything, including passwords, in plaintext, so it's insecure. SSH replaced it.
Under the hood
No encryption makes it unsafe over untrusted networks; still occasionally used for debugging plaintext services.
Examples
Legacy remote access, debugging ports, replaced by SSH, network testing.
Think of it like...
Like shouting your commands and password across a crowded room for anyone to hear.