Key Chain, System Password?

Hello. The past few weeks, when I shut down my ibook and then turn it back on - as it loads up, it gets to a screen where it asks me my keychain password to access my "system". I've tried every password I know - and I do not know what this is. However, once I get it wrong for three times, it just says try again later, and then the computer works totally normal. What is with this password? How can I get it or fix it? Even under Keychain/passwords, nothing is listed for system...