Carrying capacity of the land increased - during spread of agricultural revolution in North America.
In practical terms it means the yield increased, because a new select crop produced more per acre, or because the crop or newly introduced techniques produced more frequent harvests, or because better fertilizers and other agricultural techniques were used to increase yield outright.
The net result whichever way is you get to feed more people per man-acre worked, hence [population|yield] carrying capacity.
Carrying Capacity From Wikipedia:
The carrying capacity of a biological species in an environment is the maximum population size of the species that the environment can sustain indefinitely, given the food, habitat, water, and other necessities available in the environment. In population biology, carrying capacity is defined as the environment's maximal load
and Agricultural Revolution is:
An agricultural revolution or agrarian revolution is a period of transition from the pre-agricultural period characterized by a Paleolithic diet, into an agricultural period characterized by a diet of cultivated foods; or a further transition from a living a more advanced and more productive form of agriculture, resulting in further social changes...
...so the line you are referencing is talking about the increase in the maximum population that can be supported by a given region due to better farming techniques or technology.
Reading Guns, Germs, and Steel?