Sync across devices keeps your phone, tablet and computer the same: widgets you add and data you record on one device show up on your others. It's a Pro feature, and each device needs to be signed in to the same account.
To turn it on: open Me › Sync across devices and tap Turn on sync. You'll get a recovery code first. Keep it somewhere safe.
To add a device: sign in to the same account on the new device, open Me › Sync across devices, and choose how to join:
When you sign in on a computer by scanning a code, you can also tick an option on your phone to add that computer to sync.
What syncs: dashboards and their widgets; your installed tools (tools from the Store download automatically on the new device, and ones you made sync their content); data recorded by widgets; and reminders.
What doesn't sync: theme, language, columns per dashboard, and home screen widgets. By default, credentials sync only their name and the websites they're used for, not their value. Turn on Sync keys and credentials to sync the values too, encrypted. Services you connected by signing in need to be connected once on each device.
If your membership ends, sync pauses and what's on each device isn't affected. The cloud copy is kept for a while (the page shows until when), and sync picks up again when you renew.
At the bottom of the page, Turn off sync on this device doesn't affect your other devices. Delete cloud data stops sync on every device; each device keeps what it has.