The new design emphasizes Twitter’s header photo feature, which places a large picture the user chooses atop the profile page and behind the user’s profile picture.
“If you don’t upload a header photo by then, you (and everyone else) will only see a default grey image on your page. That’s not fun! To get inspired about what you can do, check out this video to see how to make your profile a little more “you”, less generic. Have fun out there”,Twitter said in a blog post.