You can gain experience (stars) and achieve different levels in the game when the star is filled up.
When you achieve an odd level you receive coins and when you achieve an even level a new free pack becomes available.
The free packs (rewards) can be downloaded from the Free section (under all sections) in the Puzzle Shop. To find it please open the app, open Puzzle Shop.
In the bottom left corner you'll see a Menu button (three horizontal lines). Tap on it and scroll down to Free category.