This device is a burglar alarm. It uses a photoreceptor to detect if a door is open. If the door is opened, more light is let in, which trips the photoreceptor threshold and lights up the LED. The LED event is sent to the Particle Cloud where it triggers a text alert to your phone (this last part is represented by the LED lighting up). The device has a switch to turn the alarm on and off. When you leave the home, you turn it on; when you arrive back home, you turn it off.