What’s the law of diminishing marginal utility?

In economics, the law of diminishing marginal utility states  that, as a person increases consumption of a product, while keeping consumption of other products constant, there is a decline in the marginal utility that person derives from consuming each additional unit of that product.

 In economics, the law of diminishing marginal utility states that the marginal utility of a good or service declines as it's available supply increases. Economic actors devote each successive unit of the good or service towards less and less valued ends. The law of diminishing marginal utility is used to explain other economic phenomena, such as time preference.