Twitter is testing the ability for you to watch YouTube videos without leaving the Twitter app. Video has long been something that Twitter wanted content creators to post natively - it has also toyed with video creation tools itself like Periscope and Vine, both of which it bought and has subsequently discontinued.
YouTube links posted in tweets to date have just opened videos in the YouTube app. While that probably encourages more video to be posted on Twitter natively, it's not a great experience for the user. And many people probably didn't return to the Twitter app as a result.
The move to show YouTube videos in-app could be seen as an admission that Twitter just wants to curate video content rather than being a proper home for it.
Twitter is also developing some other media features at present including the ability to see larger images on the timeline and share higher-resolution images up to 4K, too.
The social network has been busy recently, with new features coming to Fleets (though in our experience they seem to be used by brands more than individuals) and Spaces, Twitter's Clubhouse-like audio chat channels. There is also an 'undo' button being tested, too as well as possible subscription channels.