Leviticus 22:10 "A tenant of the priest or a hired servant may not eat of the holy [food]"

A Priest may have hired a person to work for him on a long or short term basis, or he may have rented out a room in his house to an individual or family.

This Negative Mitzvah cautions us that even though these people live in his house, they are still forbidden to eat from the Terumah.