scratch the surface

/S K R AE1 CH DH AH0 S ER0 F AH0 S/
verb
  1. 1

    To barely begin; to see or do only a fraction of what is possible.

    โ€œThis beginners' course only scratches the surface of the subject.โ€

Source: https://en.wiktionary.org/wiki/scratch%20the%20surface