The keys are an item that allows you to obtain the prizes hidden in Milly's treasures in the Treasure Hunt game mode. When you enter the game mode, you will see your balance of this item next to a key icon.
You can find them scattered in Mountain Track. Along the track, you will notice that there are certain squares that contain a floating key. If you land on one of these, and answer the question it offers correctly, the key will be added to your balance, which you can verify at the top of the game.
Once you have obtained the required amount, you will need to go to the Treasure Hunt section to open the chest that you wish.
We invite you to visit the Treasure Hunt article for more information.