The update auto populates your timeline with photos and Vine videos. Before, you had to click through the tweet to see the media that was being shared.
It takes up a lot of "screen real estate" so if this isn't your cup of tea, here's the simple two-step way to turn it off.
First go into your settings:
Screenshot
Then, swipe back on "Image Preview". (Green means on, no green means off.)
That's it!