A more thorough explanation:

A villein in gross is a type of villein who is entirely subject to a lord or attached to a manor, but free in relation to all others. They were essentially serfs, meaning they were bound to the land and could not leave without permission from their lord.

For example, during the time of the Domesday Inquest, which was shortly after the Norman Conquest, about 40% of households were marked as belonging to villeins. They were the most numerous element in the English population. Another type of villein was the villein regardant, who was annexed to the manor of land.

These examples illustrate how villeins were an important part of medieval society, and how they were tied to the land and their lords. Villeins in gross were particularly vulnerable, as they could be transferred from one owner to another by deed, essentially becoming property.