I'm sure you must be joking with this question, but here is the succinct answer :
Because of the U.S.'s prominent role in the Cold War, the President of the United States was often dubbed the "leader of the Free World", particularly within the United States itself. The phrase implies that the U.S. president, as leader of the principal democratic superpower, was by extension the leader of all the worlds’ democratic nations. Although it had a Cold War origin, it is still used to describe the U.S. Presidency today.[1]
Source
http://www.answers.com/topic/free-world