It was a remarkably resilient, self-perpetuating system, based largely on the use of land and other renewable resources, all ultimately powered by sunlight. Wealth was primarily derived from land and the various uses of land. Here is a simplified org chart showing the pecking order of a medieval society.
But all of that changed when feudalism was replaced with capitalism. What made the change possible was the exploitation of nonrenewable resources, the most important of which was energy from burning fossilized hydrocarbons: first peat and coal, then oil and natural gas. Suddenly, productive capacity was decoupled from the availability of land and sunlight, and could be ramped up almost, but not quite, ad infinitum, simply by burning more hydrocarbons. Energy use, industry and population all started going up exponentially. A new system of economic relations was brought into being, based on money that could be generated at will, in the form of debt, which could be repaid with interest using the products of ever-increasing future production. Compared with the previous, steady-state system, the change amounted to a new assumption: that the future will always be bigger and richer—rich enough to afford to pay back both principal and interest.