I want you to understand what I said.
What does this sentence mean?
It's what I want that you understand what I said. (Referring to the present)
It's what I want that you will understand what I said. (Referring to the future)
Either 1 or 2 according to context.
I think the sentence means 1 or 2 according to context.
If the sentence is written to describe your present understanding about what I said, I think option 1 is correct.
(And I know sentence 1 and 2 are ungrammatical because "want" does not normally take a clause as an object, but I couldn't figure out better sentences, so I reluctantly wrote the sentences to ask this question, but I guess native speakers will easily understand what the sentence means.)