After a user has entered their username and password (step 1) to login to a website, Google Authenticator is a two-step authentication system that uses a software token on a device such as a smartphone or tablet for additional authentication (step 2).
The process to use it, if your website has integrated Google’s 2-factor authentication is :
1. Install the “Google Authenticator” application on your Android/iPhone device.
2. Login to your desired website and enable two-factor authentication on your account.
3. Launch the application on your phone.
4. Point your phone at the QR code on the screen or enter the authenticator code manually.
5. Enter the 6-digit one-time verification code during your login.
Lost or re-installed phone without a backup
If you reinstall your phone, or if it’s lost or stolen, then you need to re-add your account back into Google Authenticator.
1. Launch Google Authenticator
2. Set up account / Manually add an account / Enter key provided
3. Authentication ID : Enter name of Exchange
4. Authentication key for two-factor : <16 digit alpha-numeric>
https://support.google.com/accounts/answer/180744?hl=en Good overview of 2-step verification
https://support.google.com/accounts/answer/1066447 Installing Google Authenticator