Hi zrod,
Tire roll out is how many inches the tire travels in one revolution , tire stagger is the difference of the circumference of tires, they kind of go hand in hand. Drag racers are more concerned with roll out because they use that to help reaction times in the staging beams with the front tires and obviously they want the same roll out between the rear tires to make sure the car goes as straight as possible ( NO STAGGER ) Us circle track guys care more about stagger to help the car rotate in the corners. The more stagger in the rear , RH tire bigger than LH tire will loosen the car up. Less stagger will tighten the car.
Scott