MPP Tracker is a device that monitors the input voltage and current and changes the resistance of the input circuit the way that it maximizes the peak power. MPP stands for Maximum Peak Power.
They are often parts of Inverters, however they can also be sold as standalone devices.
Designing an MPP Tracker
This project documents how to build an MPP Tracker: https://libre.solar/hardware/mppt-2420-hc.html