It's no secret all that certain countries (cough, China) sensor their internet. Well in an effort for twitter to make themselves expand into those countries, they are going to allow the censorship of tweets for countries that have censorship laws.
But some censorship laws are so bad that Twitter won't compete in those countries. That would most likely be China. They won't even allow Twitter to be there. But even it if was allowed there, their censorship in that country is to severe. The video below, thanks to AllThingsD, explains this article. And for a shortcut, 38:17 is the place to be if you want to hear about the China talk.