It's Tweet-ception.

The company's latest update lets Android and iOS users embed tweets within tweets, though they can only be seen through the mobile app (as opposed to on the web.) So instead of screenshotting a tweet or manually RT-ing something that you want everyone else to see, you can now grab the link to the tweet and have it show up in your post.

Like so:

Seems like a useful feature, but something that won't really kick in until it's working for the desktop site.