Retweeting something on Twitter amounts to taking that person's tweet and sending it out again on your own personal feed. To add something to the message, users have had to do things the old-fashioned way—beginning a tweet with RT and quoting the text manually.
Now Twitter has improved the experience, embedded retweets the way it does photos.
— Twitter (@twitter) April 6, 2015
This feature is rolling out now on the web and on iOS devices. What about Android? Well, for the third time in the past couple of weeks, Twitter says support is coming sometime in the vague future.
As some commenters have pointed out, Android users can view the new embedded retweets. We just can't create them ourselves yet.