When I start up my MacBookPro/osx/snow leopard , I get this error with yahoo messenger..
What is this keychain thing? Is this some Mac thing?
Any links to explain this concept and how to resolve it?
Keychain is where Mac OS X saves stored passwords and other secure information. Without more information, I cannot debug your problem further, but it sounds as if Yahoo! Messenger expects to find a saved password for your account with them in your keychain, and is not finding it. The program Keychain Access, found in /Applications/Utilities, can help you view (and edit) keychain entries.
To fix this:
That should fix it!