Under the feudal system, a fief was a piece of land. This is short for fiefdom.
Words that go along with fief are vassal and feudal lord; the lord (kind of like our landlords) owned the fief and the vassal was subject to all of his rules. If you were the lord of a fief, your tenant was your servant. If you were to work on someone’s farm but were paid nothing, it would be like you were working a fief. The word is not used much these days — though people still don’t like landlords much.