The specialised preamp that boosts and equalises a turntable's tiny signal — different for moving-magnet (MM) and moving-coil (MC) cartridges.
A turntable cartridge produces an extraordinarily small signal that also has the RIAA equalisation curve baked in at the cutting stage. A phono stage does two jobs: it amplifies that signal up to line level, and it applies the inverse RIAA curve to restore a flat response.
Moving-magnet (MM) cartridges output a healthier signal and work with almost any phono stage. Moving-coil (MC) cartridges output far less and need much more gain (and the right loading), which is why MC-capable phono stages cost more. Many amplifiers include a basic MM phono input; a good standalone phono stage is one of the most worthwhile upgrades in a vinyl system.