Answer by taking a quote from the following article:

Manorialism was an essential element of feudal society. It was the organizing principle of rural economy that originated in the Roman villa system of the Late Roman Empire, and was widely practiced in medieval western and parts of central Europe. It was slowly replaced by the advent of a money-based market economy and new forms of agrarian contract. Manorialism was characterised by the vesting of legal and economic power in a Lord of the Manor, supported economically from his own direct landholding in a manor (sometimes called a fief), and from the obligatory contributions of a legally subject part of the peasant population under the jurisdiction of himself and his manorial court.

The term is most often used with reference to medieval Western Europe. Antecedents of the system can be traced to the rural economy of the later Roman Empire (Dominate). With a declining birthrate and population, labor was the key factor of production. Successive administrations tried to stabilise the imperial economy by freezing the social structure into place: sons were to succeed their fathers in their trade, councilors were forbidden to resign, and coloni, the cultivators of land, were not to move from the land they were attached to. The workers of the land were on their way to becoming serfs.  Several factors conspired to merge the status of former slaves and former free farmers into a dependent class of such coloni: it was possible to be described as servus et colonus, "both slave and colonus". Laws of Constantine I around 325 both reinforced the semi-servile status of the coloni and limited their rights to sue in the courts; the Codex Theodosianus promulgated under Theodosius II extended these restrictions. The legal status of adscripti, "bound to the soil", contrasted with barbarian foederati, who were permitted to settle within the imperial boundaries, remaining subject to their own traditional law.  As the Germanic kingdoms succeeded Roman authority in the West in the fifth century, Roman landlords were often simply replaced by Germanic ones, with little change to the underlying situation or displacement of populations.  The process of rural self-sufficiency was given an abrupt boost in the eighth century, when normal trade in the Mediterranean Sea was disrupted. The thesis put forward by Henri Pirenne, while disputed widely, supposes that the Arab conquests forced the medieval economy into even greater ruralization and gave rise to the classic feudal pattern of varying degrees of servile peasantry underpinning a hierarchy of localised power centers.

How much of society was involved in manorialism?
it was possible to be described as servus et colonus, "both slave and colonus".