Serfdom is semi-slavery. We call it "maaorjuus"; "orjuus" being slavery. Direct translation is land slavery (as you were "tied" to land plot).
And the thing why serfdom comes up relating to Russia, is that in Russia serfdom lasted so much longer than in the rest of the Europe.
"In 1649 up to three-quarters of Muscovy's peasants, or 13 to 14 million people, were serfs whose material lives were barely distinguishable from slaves. Perhaps another 1.5 million were formally enslaved, with Russian slaves serving Russian masters.[20]
Russia's 23+ million privately held serfs were freed from their lords by an edict of Alexander II in 1861. The owners were compensated through taxes on the freed serfs. State-owned serfs were emancipated in 1866."
Serfdom - Wikipedia, the free encyclopedia