Anyway, this graph shows the relationship between energy inputs (work) and energy output (typically food, but also shelter, children, luxury goods, etc.) for a given variety of human technology/economic organizational structure.
(Note that the graph is not to scale and only a conceptual representation of the idea.)
So for example, in a hunter gathering society, inputing more energy by hunting more often will reward people with more food, but only up to a point. As game becomes scarcer, hunters bring home less food, and eventually you eat all of the animals in the area and are actually getting less out of hunting than you’re putting into it.
Even at its maximum efficiency, a hunter-gatherer society simply can’t (in most environments) obtain much food and can’t support many people.
Growing food takes much more energy, but the results support far more people.
Modern industrial societies take a ton of energy to run, but also support billions of people, cities, etc.
Of course, even modern industrial societies still need to be careful about that right-hand side of the curve.