SSH is a protocol for secure remote login and other secure network services over an insecure network. Most hosts provide SSH access with your hosting plan. SSH access allows you to log into a command prompt, perform common administrative tasks, and execute commands just as if you were sitting at the server itself. You can run additional tools such as Git, Composer, WP-CLI, etc.
Usually, your host will have information on how to connect to your site through SSH access. A general overview is provided below, but specifics may vary from host to host.