Secure Shell (SSH) is a protocol used for securely connecting two computers.
A terminal client can use SSH to connect to (and control) another computer or device, like a smartphone or e-reader or cable box or any other computer or computer-like device that can be accessed via SSH.
SSH can also be used by FTP clients in order to move files between computers.
(You can login to another computer using a username and password. But ideally, you should what is called an SSH key.)