SSH is one of the main key services in Linux/Unix based systems. SSH represents Secure Socket Shell. SSH protocol is used to access the remote server/system with an encrypted method of login using the default TCP/IP port 22 or a custom based port number.
This service is mainly used by System and Network Administrators to execute commands and manage servers/systems. We can also say SSH is a replacement for Telnet, rlogin, and rsh which is a completely insecure protocol to connect the system across the network.