A lot of historians throw around the term ‘agricultural surplus.’ By this they mean food which the farmers and their livestock don’t eat, and which can be used to feed stonemastons and metalworkers and scribes and priests and gentlemen farmers. In this theory, societies have to find a way to produce a larger surplus before they can produce things historians like such as books. I think this term is one of the terms which historians borrowed from economists in the early 20th century.
At first the idea seems harmless enough: if a family needs 20 bushels of barley to feed itself and its animals and have seed for next year, and they harvest 30, they will probably trade 10 for something else or use it to fatten stock. But in the real world there is rent and taxation. And when you look at the science of nutrition, you find that there is a range in the amount of food that farm workers eat. At the low end, they can’t work very well, lose most of their children, and die young of chronic diseases or infections which their weakened body can’t fight off; at the high end, they have a varied diet, grow taller and stronger, and can be pretty sure of having surviving children. Its not actually the case that people need a certain number of calories of Generic Food ™ a day, above which they just get fat and below which they die. Taxes and rents often come out of this margin in between. And it is usually taxes and rents which pay for the stone buildings, the scholars writing treatises on ethics, and the beautiful silver cups.
Also, this kind of talk often assumes that production was limited by technology. But when we look at actually existing small farmers, we see that peasants often have the capacity to produce more than they do. But if they are not sure that they will be able to buy food if their cash crop fails, or there is nothing they want to buy, they often prefer to grow enough for their needs then sit under a tree or organize a nice dance with some dirty songs about the reeve of the next village. Growing half a dozen staple foods is not as efficient as focusing on two or three cash crops, but it does let the family be sure that they will have something to eat next year. There is a lot of talk about population growth leading to too many farmers and not enough farms, but in colonial Virginia, vast tracts of land were deliberately kept out of use because the people who had claimed them wanted to keep rights to the property. Today, First Nations in Canada and villagers in Africa often notice that national parks are planted on excellent farmland and run by people who want to keep the traditional users of the land out. Sometimes, the real land shortage is because a few rich people lay claim to more than their share and use it in a way which is not very efficient, not because commoners thoughtlessly have too many babies.
I think that Geoffrey Kron would argue that this idea is an idea from the perspective of capitalists and state bureaucrats, who want to extract as much from the farming operations as they can for their own purposes, not the perspective of small farmers who would like better roads and maybe a new church. The people asked to live off “necessary subsistence” often want more than the people defining “necessary subsistence.” James C. Scott would talk about the ways bureaucrats try to simplify reality so it can be measured and modelled, even if the new simplified system works much worse than the old complex one.
How much of what the farmers make goes to feed other people is not just an arithmetic operation. It tells you about the structure of the society. A society can have prosperous farmers who work together on communal projects, or it can have starving day labourers and a handful of rich estate owners who let a few clever and amusing people eat at their table. Both have the same amount of food going to people other than farmers, but they are very different societies to live in.
Further Reading: I don’t know of a good overview of how this spread into the way historians talk, but on folks like David Hume and Adam Smith there is an article Anthony Brewer, “The Concept of an Agricultural Surplus, from Petty to Smith,” Journal of the History of Economic Thought 2011 pp. 487-505 https://doi.org/10.1017/S1053837211000290 Colin Clark and Margaret Haswell’s The Economics of Subsistence Agriculture (1970) talks about how much food consumption varies between different 20th century peasants, and how at a certain level families start to keep oxen or eat meat regularly.