When you start the game, you'll have a choice to begin the game and link it to a guest, Google or Facebook account.
It is highly recommend that you link your game to a social account so your save progress is kept safe. Linking your game to a Facebook or Google account allows you to review your play history and save your progress. Also, if you ever switch devices, you'll be able to play from your existing save game progress by logging into the relevant social account you used previously.
Guest accounts allow you to play the game, but not all features that save your progress will be enabled.
At any time, you can link your guest account to a Google or Facebook account by tapping the icon on the top right corner.