Whichever one you mean, of course!
If you understand biology (the whole field of biology as a unit, the grand plan of biological theory, how it all fits together and makes sense), say "I understand ..."
If you are understanding individual lessons the teacher gives, one after another, having more successful experiences in comprehending what you read in each chapter in your biology text, generally having more "Aha!" moments, then say "I'm understanding ..."
"I understand it better" = I comprehend it in its totality more than before. I'm better able to explain it to myself and others. I know it better.
"I'm understanding it better" = I am having more "understanding experiences" than before with regard to it.
Similarly,
"I have a headache" vs. "I'm having one of my headaches again", the latter emphasizing the recurrence of headaches and that this one is the one currently being experienced.
The progressive in these cases often suggests something recurring.