As more resource is being added, it is not contributing to total output. It will reach a point where additional resources will actual cause total output to decrease. As additional quantities of the variable resource are combined with the fixed resource, a point is eventually reached where each additional unit of variable resource yields a smaller marginal product. A cost paid even if no output is produced. The cost of variable resources.
Marginal cost is a key to economic decisions made by the firm.
Marginal cost indicates how much total cost will increase if one more unit is produced or how much total cost will decrease if production is reduced by one more unit. The long run is not a succession of short runs. It is best thought of as the planning horizon. No cost is fixed in the long run. Every cost is variable.