The error "Keychain "login" cannot be found to store ..." 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?

Thanks,
To fix this:


Run Keychain Access (found in /Applications/Utilities)
click "Keychain Access" in the menu bar at the top and select "Keychain First Aid". 
Run the repair and then run verify.


That should fix it!