The "Mandate of Heaven" as you said, refers to the Zhou Dynasty's justification for it's leadership. The justification was that the gods had given them power to rule. At the time there was a system commonly referred to as the, "dynastic cycle" which involved an emperor gaining power (and hence the mandate of heaven). He would rule well but eventually he would become greedy and start to tax the peasants more and more. The peasants would overthrow him and install a new mandate of heaven.
