When people say this, they are referring to the mixture of cultures that can be found there. However, these cultures are not freely mixed. It seems as if people talk a different way or look a little different than the people around them, they are looked down upon. Despite this, it is impossible to avoid contact with other cultures in America.
Some people even try to become a part of more than one culture.
There are some advantages to such a situation, but there are also disadvantages. Differing cultures in America can be referred to as exclusive clubs. The requirements for membership are that you look a certain way, talk a certain way, and act a certain way. People are born into a club and are taught by its members the way to act. They can try to join it, but they will most likely be unsuccessful.