First, a little motivation for studying what can seem an abstract and quite boring concept: marginal products. When the communists took power in China they forced farmers to adopt new, "scientific" methods for improving rice yields (so you know, almost anything that broke with tradition was considered "scientific"). These were no dumb men. You can't be stupid and take over one of the largest countries in the world by defeating a U.S.-backed government. Some of the new agricultural methods they forced farmers to use was, however, for those who know anything about agriculture, very dumb. They assumed that adding more fertilizer increases yields by the same amount, no matter how much fertilizer had already been applied. The peasant farmers forced to do this knew it wouldn't work, and it didn't, helping to contribute to one of the largest famines to ever occur.
What the Chinese communists didn't understand was the concept of marginal product and how it changes as the use of an input is increased. To prevent such famines from occurring again, it is important that you do understand it.
Instead of a video I am posting these slides I used in class.
Available here.