A failsafe is a device that you install inline with one of your servos. Typically, we install them on the throttle channel. A failsafe monitors radio signal and battery strength. If for any reason the reciever loses the signal from the transmitter, detects interference from another transmitter on the same frequency, or detects the reciever batteries are about to die, it will move the throttle servo to a preset position. Usually this position is a braking position. So, if you lose radio signal, the failsafe will automatically apply the brakes so the car doesn't take off at full speed out of control.