This is a very open-ended question that does not seem to have one correct answer. If this is being assigned to you for a class, it seems likely that there is a specific answer that you are supposed to give that comes from your text. It would be best if you would check for such an answer since we cannot know what text(s) you are using or what your instructor means by this question.
One way to look at this question is to think of citizenship in its legal and technical form. In that case, the answer would center on legal rights. Citizenship would mean the legal right to live in a given country and to be protected by all its laws. Citizenship would be important to human beings in that it would allow them to be seen as the legal equals of all other people in the country where they live.
Another way to look at the question is to think of citizenship in more philosophical terms. Here, we would be thinking about what it means to be a citizen of the human race or a citizen of the world. In that case, citizenship could be defined with respect to human rights. A citizen would be someone who was entitled to human rights but who also had the responsibility to respect the human rights of other people.
Thus, there are different ways to look at this question and a variety of different ways in which to answer it. I suggest that you check with your instructor and/or in your text to see what answer is expected of you.