A shell (like a unix terminal) that gives you a secure way to do things, such as remotely logging in to another server, or executing commands remotely. With older programs like telnet, you had to login in the terminal passing your credentials without encryption. Secure Shell encrypts the stream to attempt to mitigate someone other than you from obtaining your credentials. SSH uses public-key cryptography to authenticate the remote computer and allow it to authenticate the user, if necessary.

https://en.wikipedia.org/wiki/Secure_Shell