My English skills are poor. Which of these is correct to say?
"I can't understand what does it mean"
"I can't understand what it means".
"I can't understand what it means" is an indirect question. You need to use a direct word order, like in a statement, after what/when/where/if/etc.
"Can you tell me where the bank is?"
"I was wondering how you did that. "
If you really want to use your first example, you need to separate it into two distinct sentences: "I can't understand. What does it mean?"