As a noun ssh

is a program and protocol for securely logging in to and running programs on remote machines across a network, with encryption to protect the transferred information and authentication to ensure that the remote machine is the one desired.

As a verb ssh

is to use ssh to connect to a remote computer.

As an initialism SSH

is secure Shell a networking protocol by which two computers are connected via a secure channel.

As an interjection shh is

requesting silence. Asking people to keep silent.