I recently read the excellent book The Inheritance of Rome. It covers the period between 400 and 1000 AD, i.e. the Dark Ages.
Until reading it, I hadn't really considered what it was like to be a serf in the feudal era. You learn about the period in World History class in high school, but mostly just mechanistically — how the feudal system worked, etc.
It's hard for the modern reader to understand just how oppressive the situation was.
Suppose you're a peasant and you want to improve your life. The local lord demands too much grain in taxes. You have no ability to vote him out of office — even speaking out against him could cost you your life.
Do you hold secret meetings to organize some kind of resistance? If one person betrays you, it's game over.
Maybe you publish an anonymous letter to get your ideas out for how things should be run? No, you can't write, and your peers can't read.
If you can't change this lord's behavior, maybe move to the territory of another? No, your freedom of movement is restricted by the lord's henchmen who watch all the roads. If you make it to a neighboring estate, that lord will just turn you in — he doesn't want to encourage his own peasants to do the same.
Maybe you learn to write somehow — by befriending the local priest or something — what do you do with this ability? Can you get a message out of the village? Who will carry the letter? To whom do you send it? You don't know people in other villages. Also you won't be anonymous since the number of people who can write is so low that it could be deduced.
Suppose you decide to start an uprising against the unjust lord. You have no weapons or armor or military training. Pitchforks don't fare well against armored knights.
Somewhat the wrong era, but consider the German Peasant's War in which 300,000 peasants went up against 6,000 trained men. 100,000 peasants were killed. In one battle, 10,000 peasants were killed on one side, and only 4 knights on the other.
So that is basically the situation. You can't organize. You can't gain wealth or prestige. You can't leave. You can't fight. You are just utterly trapped. This system persisted until the Black Death caused sufficient societal disruption to enable new systems to emerge.