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