Simply "Command Line Interface"; pretty much the opposite of GUI
. Instead of buttons and icons, you're dealing with mostly a command prompt where you type in commands and the computer responds with text.
CLI is commonly associated with "experienced" users, and this is mostly true, but there are plenty of CLI tools that are very easy to use. The usefulness of the command line is one of Linux's best features. New users should try not to be intimidated and experiment with the CLI when possible, you'll often find that it is faster and in fact, simpler.
Here is a great introduction to the shell, called Learning the Shell.