A serf was a person in the past, especially in medieval Europe, who lived on a lord's land, farmed it, and could not simply leave. The land was not theirs. This is close to slavery, and history books sometimes let the two sit side by side, but though actually many writers draw a line: a slave could often be sold as a person, moved anywhere; a serf was tied to that piece of land, went with it if the land changed hands, and might keep a few customary rights even while they had to obey. The life was still unfree and often hard. It was a class, not a job — whole families were serfs. You meet the word in history class far more than in daily talk, unless someone is using a sharp metaphor (wage serfs) to say workers are trapped. The Latin source, servus, meant slave, serf, or servant; English later split those ideas into different words, but the old closeness is still sitting underneath.