Utility: each good we buy provides us with some value. You may think of it as satisfaction.

Marginal utility: the increase in value or satisfaction from buying an additional good.

How much more do you enjoy two espressos compared to one? That difference is the marginal utility.

If you add up the marginal utilities of each espresso, you get your total utility.

Goods have diminishing marginal utility. Each good brings less satisfaction than the previous one.