Unencrypted remote terminal access. Predates SSH and has been obsolete for serious use for decades, though it survives on network equipment, industrial kit and consumer routers.
Everything including the password travels in clear text. There is no safe configuration. An open Telnet port on the public internet should be treated as a live incident — it is one of the primary ways IoT botnets recruit devices.
There is no benign reason for Telnet to be open to the internet. If you find it on a device you own, disable it immediately and check whether the device has been recruited into a botnet. If it cannot be disabled, the device should not be online.
The port scanner will tell you whether 23 is reachable on a host you control. Checking from outside your own network matters: a port can be open on the machine and still be blocked at the firewall, and it is the view from the internet that decides whether anyone else can reach it.
Only scan hosts you own or have permission to test.