Jason to help you understand it a bit more, the exact meaning of the calculation is the distance in MM that the car travels for one RPM of the motor.
For example a rollout of 30 means that the car moves forward 30mm and performs one RPM of the motor from TDC to TDC (if electric motors have a tdc :P )
This directly relates to the size of the tires and the car's gearing. You need to know some constants like the spur size and the internal drive ratio of the car. Usually it is only relevant to foam-tire racing.
As the other guys have said, head over to gearchart.com and fill out the page there - it will give you a great ratio chart or a rollout chart.