Just wikipedia it. Taiwan (Chinese Taipei to the Chinese) is considered by the Chinese as rightfully being a part of China, and currently a state in rebellion.
That's simplistic. Lots of islands are "in dispute" and have been for a very long time for many different nations. His accusation was a much broader one than Taiwan, and while we do enjoy some rather high quality manufactures from Taiwan, in the greater scheme of things it simply doesn't matter. My point, beyond your simplistic take on it, was that China's history is not one of aggression outside long established borders. Every now and then, they have a dust up with a neighbor, like European countries haven't?
If China wanted to steam roll a neighbor, they wouldn't need an aircraft carrier to do it.