The ratio of the change in output you get to the change in input required to get it.
Suppose I work for 10 hours and produce 1000 widgets, but in the 11th hour I produce only 5 widgets. My average productivity for the 10-hour period is 100 widgets/hour, and my marginal productivity for the 11th hour is 5 widgets/hour. My average productivity overall is 1005/11 widgets/hour, ≈ 91.4 w/h.
Suppose I work for 10 hours and produce 1000 widgets, but in the 11th hour I produce only 5 widgets. My average productivity for the 10-hour period is 100 widgets/hour, and my marginal productivity for the 11th hour is 5 widgets/hour. My average productivity overall is 1005/11 widgets/hour, ≈ 91.4 w/h.