Multiple yakuman involve hands with more than one yakuman, as the name implies. While regular yaku may be stacked in one hand to raise value, the same can be done with different yakuman, where applicable.
In terms of scoring, the number of yakuman in the hand serves as a multiplier to the base yakuman point value. As a result, extremely high scoring hands are made possible.
Some rules do not allow multiple yakuman. In this case, even the hand is composed of multiple yakuman, the score payout is capped to that of a single yakuman.