Roll out is the amount of time that it takes to break the beams. The bigger the tire the longer it is going to take for it to leave the beams. The beams are the sensors that are used to prestage and stage the car. When you roll forward the tire breaks the beam and the top amber light is illuminated. As you roll forward the second beam is the broken and the second amber light is lit. If you barely break the second beam then you have to roll further to unbreak the beam. A larger tire will remain in the beam longer so it appears the you haven't moved yet , but you are actually leaving already. Hope this is clear as it is easier to say than to type, especially if you a "pecker typer".
ops:
Oh yeah, ET is short for elapsed time or the time it takes to run the length of the track. RT is you reaction time, or how long it takes you to leave after the light turns green. RT does not affect the ET. The clock doesn't start until the front tires are out of the beams.