SSH (Secure Shell) is the protocol that lets you manage servers and network devices remotely over an encrypted connection, typing commands as if you were in front of them. It is the standard for administering Linux servers and network equipment, and it is hardened by using key-based authentication instead of passwords and by not exposing port 22 to the internet.
Related term in this glossary: RDP (Remote Desktop Protocol)
Secure Shell, SSH protocol, port 22, SSH keys, secure remote access