After testing it last month, Twitter is announcing the wide rollout of a new feature called "related headlines."
Basically, what Twitter is doing is displaying all of the articles that embed certain tweets under the actual tweet itself, on its permalink page. Twitter says that it will "make it easier to discover stories that provide more context.
For example, this tweet from NBA player Jason Collins regarding his decision to come out was embedded in articles on ESPN, FOX Sports, Mashable, and other sites. With the new related headlines feature, Twitter now shows you that on the tweet's permalink page.
For Twitter, it's a way to give users more context to every tweet. For users, it's a new way to discover news and find out exactly why certain tweets are newsworthy. For news organizations, it's a way to encourage the embedding of tweets in articles. Twitter is telling sites that the could be rewarded for embedding lots of tweets, because Twitter is basically giving a link to sites that do so.
The new related headlines feature should be rolling out to all users today.