A diode is like an electrical version of a check valve. Energy only flows one way through a diode. I wonder why they needed it?
Edit: I looked at the picture and my guess is that is a resistor, not a diode. Those are color bands if I'm not mistaken.
Diode
http://en.wikipedia.org/wiki/Diode
Resistor
http://en.wikipedia.org/wiki/Resistor