Chapter Rewards are rewards you get for completing each chapter. Each time you complete a chapter in Story Mode, you will get 24 hours worth of goo for free, up until Chapter 10. They can be found under the Shop icon. To claim them, tap on the picture of Goo next to the completed chapter(s).

You can also purchase Premium Rewards, which will give you 4500 diamonds for each chapter you complete on top of the 24 hours worth of Goo. If you purchase the Premium Rewards, they do not expire,