What is the key?

The key is a string of 16-bit encodings that you need to copy and save when you turn on Google Authentication, as shown in the following image:

 

Use the backup key to retrieve the Google verification code

Go to the "Authenticator" app and click on the "+" in the lower right corner.

Click “Enter a provided key” in the menu that appears, and enter the page shown below. "Account name" can be entered without input, "Your key" enter the 16-bit key you backed up. Select the "Time based" option and click "ADD" to successfully retrieve your Google verification code.