It tries to take account of the well being of all groups. Margin means border or stopping point. It also means very small increases or reductions in the level of a given activity. Marginal benefit is the benefit from a given unit, say the sixth unit or the seventh. If no unit is specified, marginal benefit is the benefit from the last unit of a good.
Marginal cost is the cost of a given unit, say the sixth unit or the seventh.
If no unit is specified, marginal cost is the cost of the last unit of a good. Marginal analysis recommends taking every step for which the benefit exceeds the cost. This tends to get us as close as possible to our overall objectives.