The "Keyring" is an encrypted storage area for your network passwords. When you create a new connection and save its password to the keyring, you will from then on be able to connect to it by entering only the keyring password. This can be advantageous in situations where you'd prefer not to remember numerous passwords. Additionally it can be used to avoid entering important passwords when using your device in public.
Before the keyring feature can be used, you must set a keyring password. To set your keyring password, open the left slide-out menu and select "Settings". Tap the "Security" option and then select "Password". A keyring password must be at least eight characters long.
If you've already set a keyring password and wish to change it, simply repeat the aforementioned procedure. You may be prompted to enter your current keyring password in order to migrate stored passwords from the old keyring password to the new one. If you don't want to do this or do not know the old password, check the "I don't know the old password" checkbox.