I'm looking for an adjective defining "something that is not obvious to understand".
For example, in French, I would say "hermétique", which in English is synonym to airtight.
I first thought of "subtle" (which has its equivalent in French: subtil) but I think you are looking for "abstruse", which is something that's hard to understand, something obscure. Do we have a context for that word?
Unintuitive would describe something that is not immediately apparent, but requires some study to understand or master.
Not intuitive, not easily graspable by intuition.
Here are a few: complicated, confusing, ambiguous, blurred, obscure, perplexing.
I'd go with "unclear" - not obvious, definite, or easy to understand
I just remembered another word that fits what I had in mind:
"Cryptic": having a meaning that is mysterious or obscure.