It does vary based on the region/era we're talking about. The Russian system of serfdom was much more formalized, closer to a form of slavery, where serfs were

It does vary based on the region/era we're talking about. The Russian system of serfdom was much more formalized, closer to a form of slavery, where serfs were owned by their estates. But you're right that in many other regions/periods, peasants were formally free, and legally able to live anywhere they wanted, as long as they reached an arrangement with the landowner (otherwise, of course, it would be trespassing). Since the land was all owned by a smallish number of landowners, the only way to make a living if you were not one of them was to work someone else's land, on terms largely dictated by them, so economics effectively tied peasants to the manorial lord. I'll see if I can dig up the source, but I read an interesting analysis specifically of the transition to feudalism in late-Roman Italy. As the pax Romana weakened, the city of Rome was increasingly unable to be kept supplied with food imported through smoothly functioning trade routes (especially grain from North Africa), so much of the population fled the city for the countryside. There, they had little choice but to reach generally unfavorable agreements with estate owners for permission to work on their land, since Italy was already too densely populated for American-style frontier homesteading in unclaimed land to be possible.