If you wish to state that at the present moment you understand, then you have two choices: 1) I understand 2) I have understood (the present perfect describes the present, not the past) If you wish to state that you understood at a past moment, then say 3) I understood.

“I understand” is present tense, and “I understood” is past tense, used if you are recounting [describing] an event that has now finished. 'I understand what you mean' [present] 'I understood what he meant' [past, now finished]

I understand is present tense I understood Is past tense