Answer:
When Economists say that humans make decisions at the margin they mean that decisions are made on the basis of the cost and benefit of getting an additional unit of a good/ service.
Marginal benefit refers to the additional utility that we will derive from consuming one extra unit of a good or service and factors in heavily into our decision making. We usually accept a decision if the Marginal benefit is higher or equal to the Marginal cost ( cost of the additional unit) of the good/service.
If the Marginal Cost is instead higher, the decision would most probably be cancelled.