There have been many changes in the world that Friedman discusses, but the major change is the one that give the book its title. This is the "flattening" of the earth. What he means by this can be found on page 8 of the hardcover edition of the book. There, Friedman tells us that he came to realize that
The global competitive playing field was being leveled. The world was being flattened.
What Friedman is talking about here is globalization. He is saying that globalization has allowed companies in all sorts of countries to compete against one another on a much more even basis than they once could. Things like the creation of high speed communication (internet) and the boom in containerized shipping have allowed globalization to happen. These technological changes have led to a more competitive, "flatter" world.
To Friedman, then, the major change in the world has been the "flattening" of the world.