Marginal product is simply the change in product from one level of input to another. In this case out input is labor and thus marginal product is the change in total output from one level of labor to the next. Let's go through the first couple cases as an example. At labor level 1, output increased from 0 to 8 which means marginal product is 8-0=8. At labor level 2, output increased from 8 to 18, thus marginal product is 18-8=10. Continue this all the way and you get the above table.