A command line prompt is a way of interacting with your computer without a graphical interface. While your modern Mac uses a graphical user interface, (GUI) older computers used only command prompts. The command line usually gives you more options than the GUI. Modern Macs running OS X use a UNIX foundation which you can access through the Terminal application.
Click on your "Applications" folder inside of the hard drive.
Open the "Utilities" folder and double-click on "Terminal".
Type a command at the command prompt and click "Return" on the keyboard to use the command line.
Take care using Terminal to avoid losing data from your Mac.