LED stands for light emitting diode, and can be found in nearly every electronic device. It's almost a law that every robotics project must have at least one "blinking light".

A standard LED has two leads, with one usually shorter than the other. The short lead is called the cathode, and is usually further identified by a flat spot on the plastic housing of the LED. Polarity (hooking up the + and - leads correctly) is extremely important. Just a moment of voltage applied incorrectly can destroy the LED. It is common to have a current limiting resistor in your circuit to prevent too much current from being applied to the LED. Here is a link to a handy on line calculator to help design your LED circuit.