skim the surface (of something)

skim the surface (of something)

To do, engage with, or understand something to only a minimal or superficial degree. I know you feel like you know everything about philosophy now, but this introductory course only skims the surface. Jack never felt satisfied devoting his time and attention to one thing, so instead he's skimmed the surface of a number of hobbies and interests.
See also: skim, surface