The idea of diminishing marginal utility states that, as you get more and more of some good or service, the utility of the next unit of that good or service that you get has less utility for you.
In general, economists believe that this is true. Think of it this way: if you are hungry and you receive a hamburger, that has great value to you (assuming you like hamburger). Now you get another one, but you're not as hungry as you were so it's less valuable. Now what if you keep getting more? Probably hamburger number 5 will have very little value for you over and above what you got from #4.
This makes sense theoretically. However, it is impossible to actually measure marginal utility so we can't really know for sure.