Secret Chapters will be indicated by the letter S after the name of the World as shown above.
As you explore Worlds again to find Secret Chapters, you will have the opportunity to also achieve more Challenges. It's definitely tactical and fun and you can unlock orbs and some coins as you go along!
If you want more information about Secret Challenges and the new Coop Challenges, check out this article.
Any gameplay-related question? Feel free to ask the community on our forums.