The combiner box (aka "breaker box") functions as a switch to turn the power off and combine the strings in your solar panel system together. In this case, the MNPV6 can combine up to 6 strings of solar panels into 1 wire. 6 strings in, 1 wire out.

Why is a combiner box necessary?

You cannot simply tie the strings of solar panels together and connect them directly to different components, otherwise you could start a fire. That is why a combiner box is necessary. The combiner box will go between the solar panels and the charge controller.