When you sign in for the first time from a device, you will need to enter an authentication code sent to your mobile.
You can also use an authentication application on your mobile to generate the code for you, faster and more reliably than SMS, especially when you travel or are without reception.
In the event that you lose your phone, your backup code can be used instead of an authentication code. Save it in a safe place.
Mobile Numbers
Authentication Application
Backup Code