iCloud Keychain on iOS devices
You should protect your iPhone, and hence your keychain, with a passcode. This code will be needed to access your iPhone, and to manage information in iCloud Keychain. Tap Settings > Touch ID & Passcode > Turn Passcode On. Enter a four-digit code, then repeat it to verify. Below that, Require Passcode sets a duration before your passcode is required after the iPhone locks (Settings > General > Auto-Lock) or you put it to sleep.
Keychain requires manual activation. Tap Settings > iCloud > Keychain and switch on iCloud Keychain. You'll be asked for your Apple ID. If you’ve already set up Keychain on another device linked to the same iCloud account, you’ll need to approve your iPhone from there, or provide the security code you set during that process. If you opt to approve access to your keychain using another device, you’ll receive a notification on it.
To save passwords ensure Settings > Safari > Passwords & AutoFill > Names and Passwords is on. In Safari, go to a web page that requires a user ID and password. Submit yours. You’re given the option to save them, but you can opt not to. With iCloud Keychain enabled, the credentials are synced to your authorised devices and you won’t have to type them. (When creating an account, ‘Suggest password’ appears above the keyboard.)