Stands for "Secure Shell." SSH is a method of securely communicating with another computer. The "secure" part of the name means that all data sent via an SSH connection is encrypted. This means if a third party tries to intercept the information being transferred, it would appear scrambled and unreadable. The "shell" part of the name means SSH is based on a Unix shell, which is a program that interprets commands entered by a user.
Because SSH is based on a Unix shell, standard Unix commands can be used to view, modify, and transfer files from a remote machine once an SSH connection has been established. These commands can either be entered manually using a terminal emulator, or may be sent from a program with a graphical user interface (GUI). This type of program translates user actions, such as opening a folder, to Unix commands (cd [folder name]).
To log into a server using SSH from a terminal program, type: ssh [servername] -l [username].
The "-l" indicates you are logging in with a username, which is required by most SSH connections (otherwise, it wouldn't be very secure). If the login name is recognized, you will be prompted to enter a password. If the password is correct, your SSH connection will be established. To end the SSH session, type "exit" followed by the Enter key.