While exploring the stages, you may encounter Dragon Balls. These mystical titular items will allow you to summon Shenron, the Dragon that will grant any wish. You can check the Dragon Balls you have in your possession by selecting the Shenron option in the main menu. Fortunately, you can also tap on the empty slot to know where you can find the missing dragon ball. In case you missed the chance to get a Dragon Ball in one of the earlier stages you've completed, simply attempt to clear the same map again until you get the dragon ball. Once you've used a wish, the Dragon Balls will be scattered and their whereabouts will change as well.
Dragon Ball Wishes
Wish List Source: Reddit