What is the green light for on the rear of NHRA Pro stock cars?

It actually goes beyond that. They position video to capture the tree and the green light on the bumper. The green light goes off when the driver launches the car (lets out the clutch or releases a line lne-loc). They later slow the video way down and see how long it took the car to move off the line compared to when the tree goes green. Of course driver reaction time is figured in, but basically what they are doing is measuring the car's reaction time.