This is a Pixhawk Passive Switch. The switch is used to enable/disable the outputs to motors and servos. It also features a status LED light.
Features:
Constant blinking ? the system is initialising
Intermittent blinking ? system is ready. Press the safety switch to enable output to the motors.
Solid ? Pixhawk Passive Switch has been pressed. Motors and servos are able to move once the vehicle is armed