Your question has already been answered. To perhaps better explain, you connect the red wire to a wire going to your back up lights. This will send 12 V through the red wire to your dashboard area where, as previously explained, the 12 V will tell your device to activate the video screen circuit.
I too was a bit confused when hooking up my back up camera to my Kenwood. Once it all goes together, you will realize it really is quite simple
Larry