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Keychain is a repository for usernames and passwords, as well as details of your credit/debit cards – excluding the security codes found on their backs.

Think of it as a keyring with multiple keys, each key being a set of credentials for a site or app.

Your Mac and iOS devices can store a keychain locally, but by storing it in iCloud, any passwords you save on your Mac become available on your iPad and iPhone, and vice versa.

To store your keychain online and keep it in sync among your devices, turn it on in System Prefs > iCloud (Mac) and Settings > [your name] > iCloud (iOS). You need to do this on each of your devices. You may find it’s already in use, as recent macOS and iOS versions ask during setup whether you want to use it.

If you later switch it off, you get the option of deleting the keychain’s data from your device or keeping a local copy.

The keychain’s contents are encrypted on your device, and in transit when syncing. On a Mac, to edit or delete keychain items, go to Safari > Preferences > Passwords.

On iOS, modify keychain items in Settings > Accounts & Passwords. Tap an item, then Edit, then the detail to amend, followed by Done. Changes then sync to iCloud and other devices that use the keychain.

Credit cards are managed in Safari > Prefs > AutoFill (Mac) and Settings > Safari > AutoFill (iOS).