The concept of marginal gain or loss is a concept used by microeconomic theory in the study of the behavior of firms and consumers.
Basically, marginal benefit measures the satisfaction or gain of consuming one unit more than one good. Likewise, a marginal cost is the cost of producing or consuming a unit of a good.
Let us use as a hypothetical example the consumption of chocolate. I eat a bar of chocolate because that brings me a benefit, which is to savor something that I love. On the other hand, the marginal cost of eating chocolate can be fattening.
At first I will eat a bar of chocolate and it will bring me a great satisfaction. That is, my marginal benefit from the first bar is high and the cost is low. However, if I eat one more, two more, three more bars, I'll be sick. That means that with each additional bar of chocolate I eat, my marginal benefit goes down. That is, the marginal benefit is decreasing! At the same time, with each additional bar of chocolate I eat, I'll get fatter. That is, the marginal "cost" is increasing!
This is applied to everything, do not stick to the example of chocolate! Consider, for example, the benefit and marginal cost of traveling.
In this way, as long as the marginal benefit of an activity is greater than the marginal cost, people will continue to practice the activity.
