The Secure Shell
SSH or Secure Shell is a protocol that allows a user to create a session between two computers. Through it, is possible (among many things) to access a command line interface, its useful to login to remote servers for instance.
SSH becomes quite useful for system administration tasks and even it can become part of your software development workflow (e.g: deployment). Check this nice whiteboard animation by K. Cholewa to start getting the idea:
Check this Dropbear server/OpenSSH client setup tutorial to know SSH more depth.