Current sensors allow you to read the current and voltage and pass this information to the Pixhawk flight controller, traditional power bricks do this and also provide 5V power to the Pixhawk. The Mauch Electronics Current sensors are a high quality and very accurate current sensor that require a BEC to be attached to the current sensor, the cable from the current sensor to the flight controller then provides ~5V power and current and voltage readings. Be sure to select the right Sensor / Brick for the batteries and current you are using.