Re: What is iCloud Keychain?
Essentially, it's part of a password suite that helps you create safer passwords and stores them for you so that you don't really have to remember them all the time.
iCloud Keychain keeps your Safari website usernames and passwords, credit card information, and Wi-Fi network information up to date across all of your approved devices that are using iOS 7.0.3 or later or OS X Mavericks v10.9 or later.
iCloud Keychain can also keep the accounts you use in Mail, Contacts, Calendar, and Messages up to date across all of your Macs. If you're signed in to Facebook, Twitter, Linked In, or any other accounts in Internet Accounts on OS X Mavericks, iCloud can push those accounts to your Macs as well.