Google "land taxes in medieval europe". It will likely come up with an article about england, but once you scroll past the typical google shit you will get a broader set of information on the subject.
Broadly defined, it was a way of structuring society around relationships derived from the holding of land in exchange for service or labour.
I'm a middle age Québécois and we learn about that system in secondary school when we talk about the early ages of New France. AFAIK it was however a bit different from empire to empire. In New France there were seigneurs (lords) holding vast pieces of land for the crown, called a seigneurie (lordship). Within that there were censitaires (serfs) that had to make use of the land and pay taxes.