The new feature is not functional yet, but Twitter was informing developers that it was just added and is now in the testing phase. According to Twitter representative Taylor Singletary, "In the future, we'll have a family of additional API methods & fields for handling end-user 'media settings' and possibly sensitive content."
According to Gizmodo, Twitter reps say the company doesn't intend to censor material. But according to Twitter's media policy document, the company will "remove media that might be considered sensitive such as nudity, violence, or medical procedures."
In the announcement, Singletary encouraged those who were curious about how this capability would ultimately be used to read that media policy document.
To us, this seems like a feature that's long overdue, giving users the ability to control the kind of content they or their children are exposed to, letting them use Twitter without fear of being unpleasantly surprised when they click on an inappropriate link.