A fief is the estate of a noble, who holds the land under the crown. While on his lands the noble can't be touched by the Crown's justice. It's the nobles who are supposed to look after their people, feed them in bad winters and help when it is needed, e.g. because of natural catastrophes like floods. The nobles also speak everyday justice for their people. This system is known as feudalism, and is practiced in Tortall and others of the Eastern Lands.