Drag brake is the amount of braking power applied when no throttle (or brake) input is applied. If there is no drag brake, the car will coast freely when you get off the throttle.
Many racers use drag brake as it gives a more instantaneous braking response than pushing the trigger forward on the transmitter, and it can feel more predictable as well, as the amount of drag brake will increase as you let off the throttle.
.png)