Back in September, Twitter introduced a new design for profiles with a header, which made your Twitter profile look a lot like your Facebook profile. Until now, uploading a header and switching over to the new profile has been optional. Starting December 12, it won't be any more, and your profile will be converted to the new design.
According to the blog post from Twitter:
On December 12, we're rolling this out to all users: you'll automatically get this new version of the profile on twitter.com. 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.