Utility refers to the satisfaction or benefit derived from consuming a good. The law of diminishing marginal utility states that as consumption of a good increases, the marginal utility of each additional unit decreases. The law of equi-marginal utility extends this to consumption of multiple goods, stating that a consumer will allocate their budget in a way that equalizes the marginal utility across goods. This occurs when a consumer spends their money in a way that maximizes total utility subject to their budget constraint.