Answer:
Serfs were the laborers of lowest levels in many zones of medieval Europe, however, in some, there were slaves, who were at a lower level. Serfs were not slaves but rather were not allowed to leave the land where they worked.
Their commitment with their medieval master was shared; he had commitments to them, to give a place and ensure them, similarly as they had commitments to him, to give a piece of the product, or later, cash for lease. Serfs couldn't be purchased or sold.
They had a place with the land, not the ruler. In the event that the master sold the land, they ran with it. The new proprietor did not have the alternative of moving them off the land.