OS X El Capitan: About your keychain password
Your keychain password lets you automatically authenticate apps and services stored in Keychain Access. When a user is created, the keychain password is set to match the user’s login password.
When you change the login password in Users & Groups preferences, the keychain password is automatically updated to match it.
If the keychain password does not match your login password, when you log in you must also enter a password to unlock the keychain.
The keychain password is available only to the user and not the administrator. If the login password is reset by the administrator, the keychain password is not changed. The user is asked to reset the keychain password during the next login. If the login password was reset because the user forgot the old password, the keys, passwords, and other information in the keychain won’t be accessible, and the user will need to create a new keychain.