If the light is green, your sensor is connected to the cellular network.
A red light means that your sensor is not connected to the cellular network.
Yellow means the sensor is connected to the cellular network but it’s running only on battery backup power. Your sensor may take a minute or two to connect to the cellular network when you first set it up.
If the LED is off, then the unit is off and has no power. It will be off when you receive it, but will wake up when you plug it in. If it was working before, then it means it's no longer receiving power and the backup battery is out of charge. Plug it into a good power source to get it going.