How to Use iCloud Password to Login & Unlock Mac OS X
Rather than remembering a separate password and set of login information for unlocking a Mac, OS X offers the option to use an iCloud password to login to the computer at boot, reboot, authentication, locked screens, and all login windows instead. This is a helpful feature for users who like to keep things simple and use a single login and password for all Apple related tasks on their Mac, since the Apple ID can access iCloud, the App Store, iTunes Store, Mac App Store, FileVault, and quite a bit more.