Recovery Key

What is recovery key associated with iPod touch (old version)?

When I wish to back up on icloud, it says "Your iCloud password can not be verified at this time". If I try to re-enter the password for my icloud account, it is expecting to enter the 'Recovery Key' starting with 'RK-'. I would like to know, what is this recovery key and how to obtain. My Apple Id is set with two-factor authentication, when I purchased and activated my nee iPhone 6s Plus. Please let me know the solution.