It is an acronym for "Yao Only Fan." It is a racist phrase intended to put down Chinese fans of Yao Ming, who do not otherwise have an allegiance to the Houston Rockets, the team Yao Ming plays for. Longtime Rockets fans are offended that there are people whose primary allegiance is to Yao Ming and not the Houston Rockets.
I can't believe that little Chinese guy is mad Yao did not get enough shot attempts even though the Rockets won the game! He must be a YOF, he only cares about Yao Ming getting good stats. Plus, his grammar is bad, he is probably from China, a stupid communist country.