First, I understand that you are among friends, since it is a rather infornmal question.
Second, I understand that somebody's asking you what you are having because, depending on what it is, he/she will have the same.
Third: you are (for instance) at somebody's place, and he/she is asking you whether you would like something to eat or drink.
There are probably some other meanings (natives will say), but we would need some more context to know for (more or less) sure
Student or Learner