A MPPT( Maximum Power Point Tracking ), or maximum power point tracker is an electronic DC to DC converter that optimizes the match between the solar array (PV panels), and the battery bank or utility grid. To put it simply, they convert a higher voltage DC output from solar panels (and a few wind generators) down to the lower voltage needed to charge batteries.
Compared to conventional PWM controller, MPPT technology increases the charge efficiency up to 20% and potentially decrease the power of solar array needed.