What can I do with the Chests I won from Arena?

When you have won chests from Arena mode, you can tap on the chests at the bottom of the main menu to bring up an "Unlock Chest" screen.

Here, you will be shown how many stickers you will win, along with the rarity of the stickers (indicated by the number of stars).

Below this, you can choose an unlocking method. You can tap the "Start Unlock" button which will start a timer (the length of time is indicated above the button). You can reduce this time by watching an advert if one is available (this will replace the "Start Unlock" button if the chest is already unlocking). When this timer is up, you can then claim your rewards. Alternatively, you can spend Gold to unlock the chest immediately.