To my limited knowledge of history, serfs were peasants who were bound to the land that they worked in which was owned by a landlord. How did these landlords stop peasants from migrating in the dark of the night for example? Did the serfs have labour strikes and such? Were all peasants serfs?
More can always be said on the matter; if anyone can post about serfs and relevant enforcement, please do go ahead! For the meantime, we do have a few previous posts on this matter. Both these posts look more at how a serf might become not a serf, but they by necessity also deal with how they were kept on.