Secure Shell or SSH is a secure method for remote access as it includes authentication and encryption. To do this, it uses an RSA public/private keypair. SSH uses TCP port 22.
SSH provides a command-line interface to remote systems from which CLI commands can be input. Typical devices to which you can connect are Cisco routers, switches, and firewalls running some version of IOS, NX-OS, or ASA operating system for example, or a Linux-based server, to name only a few.
SSH is also used as a connection method for network automation tools such as Ansible for example.
There are two versions of SSH: version 1 and 2. Version 2 is more secure and commonly used.
SSH is preferable to Telnet which provides similar CLI access but without the level of security and confidentiality of SSH.