The answer above me is correct. However, I would like to add that while "I see" and "I understand" are acceptable in all situations, "got it" or "I get it" are more casual and they could sometimes be taken as rude. In formal situations, I would use "I understand" just to be safe.

我知道了 can be translated as "I see.", I understand", "I get it" or "I've got it". The last one can be shortened to "I got it." or simply "Got it". You can't say "Get it." though, because that is telling the other person to understand. "Understood" is fine too. These can have other meanings too.