This is an easy one - Put simply, a diode lets current flow in one direction, but not the other. Kinda like a check valve.
A couple of things to remember about diodes - a diode will cause a bit of voltage loss in a circuit. This is referred to as forward voltage drop, and is usually around .5 - .7 volts. Diodes also have a maximum reverse voltage. If this is exceeded, the diode will break down and allow the current to flow in the wrong direction (that'd be bad, mkay?)
The polarity of a diode is denoted by a stripe on one end of the diode. Current flows into the end furthest from the stripe and passes out the other, but not the other way 'round.