How to protect your iCloud account
> Use two-factor authentication to add an extra layer of security to your iCloud account. It ensures that people can’t log in as you from other devices even if they’ve managed to get hold of your Apple ID and password. If someone tries to log into any iCloud service from a new device they’ll be asked to enter a code that’s only shown on your other Apple devices. You can switch on two-factor authentication in System Preferences > Apple ID > Password & Security.