The unit plugs into the lighter/accessory port in the car, and keeps the battery charged up with a trickle charge, even in cloudy weather.
"Batteries require a constant trickle charge for peak performance and guaranteed starts. Leaving your vehicle parked for days or weeks without use will slowly drain the batteries, which can lead to premature failure."
This is going to be most useful for those who have vehicles that sit for long periods of time and for those who don't have ready access to another vehicle for jump starts. It can also be very useful for use on boats and other vehicles that are less easily helped with a jump start.
It doesn't quite turn your car into a solar powered wonder, but you can claim that you get a portion of your car's power from the sun.