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Serfdom is an Economic Policy Card in Civilization VI.
Strategy
This Economic Policy replaces the earlier Ilkum, but unlike others which similarly replace earlier Policies, it doesn't simply update the benefit to the new era; it actually changes it! Instead of a 
Production boost, Serfdom provides 2 more Build actions for each Builder, for a total of 5 (without any additional bonuses). This allows a single Builder to almost do the work for two!
The way you use the Serfdom, however, doesn't change with respect to the earlier Policy - you should slot it, then hurry to build (or purchase) as many Builders as you need until the next change of your government structure. Note that since the Production/
Gold cost of the unit will keep going up, it will take more and more turns to build them, especially without the Production boost from the earlier Policy. This makes it more worthwhile to purchase Builders if you can afford it. Incidentally, this is not a bad idea, especially since you should have many new cities at this moment of the game, which will benefit greatly from some Builders popping up at the click of a button, without all the fuss of producing them in your older cities, and then transporting them to the new ones.
Civilopedia entry
Although forms of land bondage had been present in civilization before (such as the Roman colonate system), medieval serfdom really began with the collapse of the Carolingian Empire in the 10th century AD. In the period following, local lords needed a ready supply of agricultural labor for their manors, and the peasants needed protection against just about everybody … so the practice of serfdom spontaneously arose in various places across Europe. By the 12th and 13th centuries, serfdom had acquired a strict legal status, for serfs were not slaves and did have rights (however minimal). With the rise of powerful monarchs and central governments, serfdom became less useful, and by 1400 true serfdom was rare … except in Russia.