SSH stands for Secure Shell. It is a protocol that lets you drop from a server "A" into a shell session to a server "B". It allows you interact with your server "B".
An SSH connection to be established, the remote machine (server A) must be running a piece of software called an SSH daemon and the user's computer (server B) must have an SSH client.
The SSH daemon and SSH client listen for connections on a specific network port (default 22), authenticates connection requests, and spawns the appropriate environment if the user provides the correct credentials.