Three, four, or five scatter symbols will trigger the free spins. You will then receive seven, ten, or fifteen free spins as a result. The bonus spins cannot be reactivated.
You could notice that moon symbols start appearing on the reels during the bonus. They can appear anywhere on the screen and each has a value carved into it. Any moon that lands during a free spin has its value totaled, and that amount is then added to the overall prize payout.