[nuttle]

An encoder is a sensor that can give you information on position, from which you can get velocity and even acceleration information. In this case, we're talking about rotational position, or how far something has turned. For a drive base, this can let you know how far you've travelled, help to keep you going in a straight line, or even detect wheel slip. For a shooter, this can help you control how fast a wheel is spinning, to get more consistent shots. But this requires hooking the encoder up and writing software to take the information from the encoder and use it to control the motors. A little searching should turn up a lot more on this topic...