Coins are used in the game to purchase basic outfits for your Hatchling collection. You can also use the coins to open basic chests after you have used your keys and claimed your free chest.
You’ll get coins as a level reward when you pass the levels. In addition to level rewards, you’ll get additional coins as an outfit bonus based on the outfits you own. Each outfit will give a certain coin bonus. You can see how many coins you’ll get as a bonus from owning the outfit when you view the outfits in the Collection menu.