[iMore Question]

posted another question earlier here's another. What is the iCloud 'keychain' and how am I supposed to use it?

[LillyISabelle]

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 10.9 or later.
iCloud Keychain can also keep the accounts you use in Mail, Contacts, Calendar, and Messages up to date across all of your Mac computers. And when you sign in to Facebook, Twitter, LinkedIn, and your other Internet accounts, iCloud automatically adds your usernames and passwords to all of your devices.

Read More https://support.apple.com/en-us/HT204085

[Ledsteplin]

Also when you enter log in info in Safari, it offers the option to save that info to Keychain. You access it by going to settings > Safari > passwords. You'll need your passcode or fingerprint to get in. You can scroll down to the bottom and add more passwords or other info.