In the Middle Ages, when Crispin: The Cross of Lead is set, lowly peasants were essentially tied to the land where they worked. They weren't slaves, because they were paid for the work that they did, but they didn't enjoy freedom, either.
As serfs, they were bound to the land and the nobleman who owned it. Villeins, as they were known in England, were feudal tenants who paid dues and services to the lord of the manor in return for land. As such, they could not live anywhere else; they had to remain where they were, working the land for the benefit of the lord of the manor.
This is precisely the situation that Asta's son, or Crispin, finds himself in. He and his mother are reminded at every opportunity by the steward of the land on which they work, John Aycliffe, that they are villeins or serfs bound to Furnival, Lord of Stromford Village.
Even though Asta and Crispin, like most peasants, have never actually laid eyes on the lord of the manor—he's been fighting in France for many years—they're still expected to work his land all year round, come hail, rain, or shine.
The money that Asta and Crispin receive for their back-breaking toil is an absolute pittance. Asta receives just a penny a day—the price of a loaf of bread—whereas Crispin has to make do with a farthing, one quarter of a penny.