The law of diminishing marginal utility is that when the consumption of an article increases, its marginal utility tends to decrease.

For example, if you are thirsty and want to buy coke,

For buying the first can of coke, you get the satisfaction of 10, so its marginal utility is 10.

Then buy a second coke, because the satisfaction you get from the second coke is not necessarily as great as the first coke. Assuming that it is 8 units of satisfaction, then the marginal utility of the second cup of coke is 8.

. . .

By analogy, if you don't get any satisfaction from drinking the fifth coke, then the marginal utility of the fifth coke is zero.

So this explains the law of diminishing marginal utility, which is based on the fact that the enjoyment you get from an item decreases as the consumption of the item increases.