Most modern computers have two very different ways that you can interact with them: the Graphical User Interface (GUI) and the Command Line Interface (CLI).
Most users are familiar with GUIs. When you use your mouse to point and click on things, you are using a GUI. It is very likely that you have never used the other type of interface: the CLI. With a CLI, you use text to control functions and software on your computer.
If you really want a fast way to work on your computer to change configurations, install software, or work remotely on another computer, then the command line is often the most efficient way to do it.
A type of CLI is the Terminal. In the Sugar world we call this the Terminal Activity.