Utility: super-general term meaning whatever a person cares about. Marginal utility: incremental change in utility when some other thing changes. The more money you have (all else being equal) the less you care about having $1 more or less.
Therefore, if you make the (ridiculous) assumptions that (1) there's a fixed pot of money available and (2) different people have very similar utility functions, it follows that everyone should have the same amount. (Because transferring money from someone with more to someone with less makes more difference in utility for the person with less.) Which is more or less what communism is trying to achieve.
That's a pretty huge more-or-less.
I have found this most wonderful (if fairly lengthy) article, and thought you would enjoy having it brought to your attention.
It is so good, I think we should include it among the references in the "Politics is the Mind Killer" wiki page. But, before that, I submit it to you, and ask you: is there anything in this article that would warrant its exclusion from this site? I mean, besides the fact that it is about politics, and written by a notorious Social-democrat (And is that in itself grounds for exclusion?).