Serfdom is the system that uses serfs, or the condition of being a serf: that whole order of tying farm people to a lord's land. Rules changed from country to country and century to century, but the core stayed the same — you were not free to go. In history writing it names a real legal world, and it ended at different times in different places. Writers also stretch it (economic serfdom, intellectual serfdom) when they want to say people are trapped and must obey, even if no one is a medieval peasant. That stretch is dramatic on purpose. The ending -dom is the same piece you meet in kingdom and freedom: it turns a person or idea into a state or a system. Freedom and serfdom are opposites wearing the same kind of tail, which is a little bitter and worth noticing.