From the tremulous management of Twitter:
"Starting today, we give ourselves the ability to reactively withhold content from users in a specific country — while keeping it available in the rest of the world. We have also built in a way to communicate transparently to users when content is withheld, and why."
The statement from Twitter management says, essentially, that Twitter is afraid of foreign governments and others who try to shut down free speech around the world. That includes spurious claims of libel and defamation because someone said something on Twitter that someone in another country did not like.
Or, to put it in the Twitter management language, it will remove tweets in unspecified countries that have "different ideas about the contours of freedom of expression." In honest English, that means, countries that don't respect free speech.
Those of us who have fought the good fight regarding Internet censorship of free speech are appalled to see Twitter become a handmaiden of censors, as are many Twitter followers. See this link from the Guardian newspaper.
Incidentally, there are those who argue that Twitter's move is basically a good one for free-speech interests, but I don't buy that argument myself. Giving in morally to censors who want to shut down certain areas of speech is always, always a step down a slippery slope, in my opinion.