The marginal product of a unit of labor is the added amount of product that can be made when that extra unit of labor is used. In other words, let us say that I have three workers in my shop and we make 10 shirts per hour. Let us then say that I hire a fourth worker and we are then able to make 15 shirts per hour. In that case, the marginal product of the labor of the fourth worker is 5 shirts per hour.
As can be seen here, we obtain the marginal product of a unit of labor by taking the amount of the product made with that labor and subtracting from it the amount of the product made without that labor.