A more thorough explanation:

A villein is a person who is completely subject to a lord or attached to a manor, but is free in relation to all others. They were like serfs and were the most numerous element in the English population at the time of the Domesday Inquest, shortly after the Norman Conquest. There were two types of villeins:

• Villein in gross: A villein who was annexed to the person of the lord, and transferable by deed from one owner to another.
• Villein regardant: A villein annexed to the manor of land.

For example, if a lord owned a manor, the villeins who lived on that manor were completely subject to that lord. They had to work the land and pay taxes to the lord, but they were free in relation to all others. They could not leave the manor without the lord's permission. If a villein was a villein in gross, they could be sold or transferred to another lord by deed. If a villein was a villein regardant, they were attached to the manor of land and could not be sold or transferred separately from the land.