What does your father do?是问你爸爸是做什么（工作）的?等同于“What is your father's job?”
回答的话,比方说,在中文里,可以说,我爸爸是警察,我爸爸是律师,都是“是”,翻译成英文当然就是“My father is a policeman”和 “My father is a lawyer”了,这就是为什么回答用IS.