MPPT stands for Maximum Power Point Tracking and is an electronic DC to DC converter that optimizes the match between the solar panels, and the battery you are charging, whether that be a battery storage device or direct to a digital device. To put it simply, they convert a higher voltage DC output from solar panels down to the lower voltage needed to charge batteries.The charge controller looks at the output of the panels, and compares it to the battery voltage. It then figures out what is the best power that the panel can put out to charge the battery. It takes this and converts it to best voltage to get maximum AMPS into the battery.