marginal utility
Imagine you are hot and thirsty and someone hands you a cold glass of water. That first glass feels wonderful. A second glass is still nice. By the fourth or fifth glass you barely want it, and a tenth glass would feel like a chore to drink. Marginal utility is the name economists give to that changing feeling: it is the extra satisfaction you get from one more unit of something, not from the whole pile of it.
More precisely, marginal utility is the change in total utility when you consume one additional unit, holding everything else fixed. If drinking three glasses of water gives you a total satisfaction we can label 27 units, and a fourth glass raises the total to 30 units, then the marginal utility of that fourth glass is 30 minus 27, which equals 3 units. The word marginal always means at the edge, the next step, the extra one, never the average or the total. Notice the units are imaginary and only meant to compare your own choices; you cannot really put a number on happiness, and you certainly cannot add one person's units to another's.
Marginal utility matters because it is the hidden engine behind almost every demand curve. People do not decide how much of everything to buy in one giant calculation; they decide one more or one less at a time, weighing the extra satisfaction of one more unit against its price. That habit of thinking at the margin, comparing the next benefit to the next cost, is one of the most important ideas in all of economics.
A second slice of pizza is delicious, a fourth is just okay, and a sixth makes you feel sick. The pleasure of each extra slice, its marginal utility, falls slice by slice even though your total enjoyment is still rising up to a point.
Marginal utility is about the next slice, not the whole pizza.
A common slip is to confuse marginal utility with total utility. You can still enjoy your meal overall (total utility rising) while each extra bite pleases you less (marginal utility falling); the two move differently.