Serfdom is the socio-economic status of unfree peasants under feudalism, and specifically relates to Manorialism. It was a condition of bondage or modified slavery which developed primarily during the High Middle Ages in Europe. Serfdom was the enforced labor of serfs on the fields of landowners, in return for protection and the right to work on their leased fields.
Serfdom was practiced in Eastern Europe during the 17th century, primarily in the Polish-Lithuanian Commonwealth and Russia. Michael Stearns openly condemned serfdom and believed that it widened the divergence between western and eastern Europe, which underlay the two World Wars in the OTL.
In the Polish-Lithuanian Commonwealth, serfdom was largely "export-led", linked to economic dependence on the grain trade.
In Russia, dependence on serfdom was partly the result of a cash-poor, labor-poor economy. Bureaucrats were primarily paid in land, because there wasn't enough money to pay them. They, in turn, didn't have enough money to pay people to work that land; only the great noble families and large monasteries could actually afford to use paid labor if they needed to, and there were few of them.
When Russia began to industrialize, serfdom played a role. Many landowners essentially rented their serfs out in order to make money, at least during the months when the ground was frozen. Others used their serfs for their own industrial or commercial undertakings.