Gold Bars are the in-game currency in all King games, and can be used to buy boosters, extra lives or extra moves if you need them.
You can buy Gold Bars from the in-game shop. Just click on the green '+' symbol, next to the Gold Bar image at the top of the screen and you'll be able to make your choice of Gold Bar bundles from the store.
You'll be taken through the steps to complete your purchase, using the details saved in the App Store account held on your device.
Once you have Gold Bars in your bank, you can either tap the booster you need and you'll be shown the cost in Gold Bars, or if prompted when you run out of moves or need extra lives, the game will confirm how many you will spend if you make a purchase.