This woman predicates her question on a premise of illegal citizenship status.
"Many (illegal citizens) are fearful of voting; so if I vote, will immigration know where I live? Will they come for my family and deport us?"
Just based on this part of the transcript alone, this woman appears to be asking POTUS whether illegals can vote.
Which is why Obama answers that only citizens can vote. He says "
first of all, when you vote, you are a citizen yourself. "
He doesn't say voting makes you a citizen, but when you vote, you must be a citizen. Specifically, "you are a citizen yourself"
Just like saying "When you're president, you're a natural born citizen yourself"