You can use coins for purchasing new puzzle packs from the Puzzle Store. The price in coins is indicated on every paid pack.
If you open the preview of any pack, you will see Buy for coins button. Press it to purchase the pack.
You can earn coins by solving puzzles and watching bonus videos.
You can also ask your friends to send you coins by tapping on the plus sign next to the amount of coins and choose "Ask friends". Please note that you and your friends should be logged in the app via Facebook.
This option is available only once a day. The maximum amount of coins that one friend can send you is 10 coins.
If you need more coins, you can purchase them in Coins bank - click on the plus sign next to the amount of coins.