Sunstones are a unique, premium currency in The Wolf. You can use them to buy selected Special Offers, which are time-limited bundles that give you more value than buying the items separately.
You can buy Sunstone packs in the in-game shop, under Resources → Sunstones. Your Sunstones balance isn't shown on the main game screen. You can check it in the shop.
Sunstones unlock as you progress in the game. Your level is one requirement, but because Sunstones are tied to Special Offers, they also unlock once your account has used certain shop features. This helps make sure the offers you see are relevant and valuable to you.