You can gain experience (stars) and achieve different levels in the game when your level star in the top right corner 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 Packs for Stars category in the Puzzle Shop. To find it please open the app, open the Puzzle Shop.
In the bottom left corner you'll see a Menu button (three horizontal lines). Tap it and scroll down to Packs for Stars category.