Diamond argues that superiority in food production leads to a number of factors that can contribute to their ability to dominate others. Agricultural societies are settled societies, which makes it possible to produce more offspring. Societies that can produce agricultural surpluses also give rise to elites, bureaucrats and specialists, who can supervise the development of technology and plan campaigns of conquest. Additionally, the fact that dense, sedentary, agricultural societies also have domesticated animals means that they also are exposed to the kinds of germs that cause epidemics. In short:
...food surpluses...were a prerequisite for the development of settled, politically centralized, socially stratified, economically complex, technologically innovative societies.
The "guns, germs, and steel" that Diamond views as crucial to the dominance of some societies arose first in these agricultural societies, which gave them a major advantage when they made contact with other peoples.
Source: Diamond, Guns, Germs, and Steel, 92.