Q: what is key chain?

It's basically a password vault but built into OS X.

Back up all data before proceeding.

Launch the Keychain Access application in any of the following ways:

☞ Enter the first few letters of its name into a Spotlight search. Select it in the results (it should be at the top.)

☞ In the Finder, select Go ▹ Utilities from the menu bar, or press the key combination shift-command-U. The application is in the folder that opens.

☞ Open LaunchPad and start typing the name.

Select the login keychain from the list on the left side of the Keychain Access window. If your default keychain has a different name, select that.

If the lock icon in the top left corner of the window shows that the keychain is locked, click to unlock it. You'll be prompted for the keychain password, which is the same as your login password, unless you've changed it.

Right-click or control-click the login entry in the list. From the menu that pops up, select

          Change Settings for Keychain "login"

In the sheet that opens, uncheck both boxes, if not already unchecked.

From the menu bar, select

          Keychain Access ▹ Preferences... ▹ First Aid

There are four checkboxes in the window that opens. Check all of them. if they're not already checked. Close the window.

Select

          Keychain Access ▹ Keychain First Aid

from the menu bar and repair the keychain. Quit Keychain Access.

If you use iCloud Keychain, open the iCloud preference pane and uncheck the Keychain box. You'll be prompted to delete the local iCloud keychain. Confirm. Then re-check the box. Follow one of the procedures described in this support article to set up iCloud Keychain on an additional device.