It isn't clear to me if it your classmates who are using the word "open" to describe America. If so, perhaps you should ask them what they mean, or you should offer the ide in Chinese, and ask somebody here to translate it for you. The term "open society" was first used by Karl Popper, one of my heroes. He used it to describe a society, a culture, a country which was "marked by a critical attitude to tradition", where the government is supposed to be "responsive and tolerant". I think most Americans would believe that this was a good description of their country, though some people from other countries and cultures might disagree. So yes, it would be correct to use the term "open" to describe America's society or culture. But this is really a political judgement, and politics, like beauty, is in the eye of the beholder.