Your account
Your progress is saved to your account, so it's there on every device you log in on.
On another device
Log in there with the same email: your progress follows you, and what you do there comes back here.
Logging out makes this browser forget your progress until you log in again. It stays safe in your account.
Delete your account
This deletes your account and the progress saved to it, for good.
You're not logged in. With a free account, your progress is saved and follows you to any device.