The pun is on the word "counterfeit," which can mean either a fake or a copy. In this case, Romeo is calling Juliet a "counterfeit" because she is wearing a mask. He is also making a joke about the fact that she is crying out, "Ay," which is a common expression of surprise or agreement. The pun is a way for Romeo to make light of the situation and to show that he is not taking Juliet's rejection too seriously.