When you first open the game, you start with a guest account. A guest account is tied only to this device. If you lose the device or delete the game, your progress won't come back.
Link your account to keep your progress:
- Open Settings › Account.
- Link with Apple, Google or email.
- When you see "Your progress is in your account.", you're all set.
If you link with email, a 6-digit code is sent to your address. There is no password.
After you link your account, sign in with the same account on another device to pick up where you left off. Details: Changing devices.