The concept that if any one person had increasing marginal utility, then they would get all the resources in the world because that would be the aggregate utility maximizing solution.
This is used to demonstrate that we can consider agents having decreasing marginal utility as well as the obsurdity of the aggregate utility / social planner paradigm.
The key here is that the utility monster could justify taking the resources of others because he gained greater happiness. In the limit, this would be a world that was enslaved to the fulfillment of the monster's (person with the greatest marginal utility) desires.
(See Also: Ted Kennedy / George Bush)