The system prompt is the shell's way of saying that it is ready and waiting for you to type in a command. When the system is finished running a command, the shell replies with a prompt to tell you that you can now type another command.
The Bourne shell and the Korn shell use $ as the default system prompt. The C shell uses %. Your own system prompt may be different from what we use here. Your System Administrator can tell you how to customize or change the system prompt that appears on your screen.