A personal key is a 16-character secret code we have been historically generating as a form of two-factor authentication. Read more about two-factor authentication here
Recently we have begun the process of retiring personal keys as a form of two-factor authentication because they have not worked well for our users. In their place users now have a variety of options for a second authentication method including backup codes.
If you are creating a new account, you will not be assigned a personal key. Instead, select one of the available two-factor authentication options presented.
If you were assigned a personal key previously you are still able to use it, however next time you use the personal key you will be asked to select a new form of two-factor authentication, and the personal key option will be removed. Until you use your current personal key to sign in to your account, it will remain active and available as a second authentication method.