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.
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.