A rocket engine burns enormous quantities of propellant every second. But before combustion can happen, that propellant must be forced into a chamber already operating at extreme pressure.
That job belongs to the turbopump.
It is a compact machine capable of producing extraordinary power while spinning tens of thousands of times per minute, surrounded by cryogenic liquids, violent vibration, difficult sealing problems, and the constant threat of cavitation. This episode explains why some of the hardest engineering in a rocket is hidden inside a machine most people never see.