What is the "Setup key" shown on 2FA page?

The master key or Setup key is a special passcode reserved for certain sensitive account actions that won't need to be performed very often. For better security, we highly recommend that you save the master key but, it should be kept very secure and separate from all your other passcodes. 

The master key does the following:

• Increases the security of account password reset (the master key must be provided for a reset).

• Allows you to obtain a 2-factor bypass code for account login (the master key must be provided to get a temporary OTP bypass code).

• Allows you to bypass the global settings lock, so the settings can be changed immediately.