Marginal Rate of Substitution (MRS) is a concept used to analyze consumer choice behavior and preferences. It is the amount of satisfaction a consumer will give up by sacrificing one good in order to receive a unit of the other good. MRS is oriented towards finding individual preferences, which are essentially boundaries of tradeoff a person is willing to make. For example, if a person is willing to give up 1 unit of good A to get 2 units of good B, then his MRS is 2/1.

MRS can be used to explain the concept of diminishing marginal utility. It states that as a person gets more of a good, their marginal rate of satisfaction for having one extra unit decreases; therefore, their willingness to give up an existing good for an extra unit decreases. This concept can be explained using the law of diminishing marginal rate of substitution (MRS): if both goods offer diminishing marginal utility, consumers will be willing to give up less and less of one good for an extra unit of the other over time.

The marginal rate of substitution can also be used to indicate preferences for a certain good. If a person's MRS is higher for one good than another, that means they are more willing to give up the other good to get an extra unit of the initial good. This is an indication of the level of preference the individual has for that good.

In summary, the marginal rate of substitution concept is a useful tool for understanding consumer behavior in terms of tradeoffs and preferences. MRS indicates the rate of substitution one good has to another, as well as a person's preferences for one good over another. The law of diminishing marginal rate of substitution states that as a good is consumed, its marginal rate of satisfaction for having one extra unit decreases. This, in turn, reduces the amount of another good a person is willing to give up for one extra unit. This concept is useful for predicting the direction of tradeoff decisions, as well as understanding the preferences of individual consumers.