It is a motion sensor that awakes other Hue lights attached to it. At first I thought the sensor was also the light but you will need both the Hue Bridge plus a Hue light to activate it. Battery change is a matter of unscrewing four screw retainers at the back. For future some form of grip may be needed to pry open the rear replacement back ti get to the batteries. The unit is small enough and there are plates to fix it. The plastic cover is compact and the screw mounts have rubber seals to prevent water seepage. Overall the product has a quality feel and works a charm