Tom feels like he is of the "superior" race. He explains a book that he read that says that if they don't watch out, the white race will be "submerged" in just a matter of years. Clearly he believes that as a white man he is of the dominant race, and the fact that there is a book published about it and he can freely say these things shows that many people in society are of the same mind. Some people today still think this way, although it is socially not acceptable to say it or act like it. Fitzgerald probably included this information to show how opinionated Tom is and set the tone of society at the time.
