It sounds as if it wants the answer to your security question "what is the year of your birth".
Apple has been using these security questions for a few months now. Presumably it is to avoid theft from people guessing other people's Apple IDs and passwords. No one knows why or when anyone is asked for the answers, and like any good security system it is designed to be unpredictable.
Whatever you answered for that question, only you can know, that's the idea behind the security questions. You could have answered 1492, it really doesn't matter, as long as your answer matches what you originally supplied.
If you are being locked out because your answer is not what it is expecting, your only recourse is to contact iTunes Support and ask them to reset your password. You will have to answer some questions to confirm you are who you say you are, and then they will send you an email with instructions to reset your password and create new answers to more security questions.
Contact iTunes Support here: http://www.apple.com/emea/support/itunes/contact.html