China is an important market for Apple for two reasons. First, it is a major emerging market, which means it's recognized as a big opportunity for business growth by almost all device manufacturers. Apple's investments in the Chinese market have been expanding lately, as the company has significantly increased the number of its retail stores there. Second, China is where Apple's profitable iPhone and iPad get assembled, and as you've probably heard, there have been a number of problems lately with the factories working on Apple's devices. The latter has also been among the reasons for Tim Cook's previous visits to China, when he personally examined the work conditions in some of those factories.