The poorest class during the Middle Ages were the peasants. Within these classes there were multiple other people within them, containing different rights and freedoms.
Freemen were peasants, except they had some rights. They were free and were not enslaved and in some cases could even own land. Many Freemen lived a pretty good life, despite being in the poorest class.
Serfs, or villeins, had no rights. Serfs did labor work in exchange for a bit of land that belonged to the tenant. They did not get any money out of this, they would just work for protection and housing.
The last people that belonged to the peasant class were the slaves. Slaves were bought and sold and used for labor, they had terrible living conditions and had no rights whatsoever.