The law of diminishing marginal utility states that

The law diminishing marginal utility states that as the consumer is consuming more and more units of a good then the utility obtained from the consumption of an extra unit of a good goes on diminishing. This is because of the satisfaction obtained from the units of a commodity, as a consumer would obtain higher satisfaction from the first unit and obtain less satisfaction from the next unit of a commodity.

For example; If a person is hungry and he wants to eat chapati. So, the first chapati will give him the maximum satisfaction as compared to the second chapati.