Technically, password autofill for third-party apps has been available for a couple of years now, but developers had to enable the feature, and not a lot of apps actually did. With iOS 11, however, Apple will enable the feature for all apps by including an extra key on the keyboard when it detects a login screen.
Sign in with a tap
After installing iOS 11, when you need to sign in to an app that has its login credentials saved in Apple's password manager, your iOS device will suggest login info. To sign in, tap on the saved information and you're done. If you have multiple accounts, you can tap on the key icon to view more account details.
To add, edit or delete login info using the expanded password manager, open Settings > Accounts & Passwords > App & Website Passwords. If you've previously let Safari save any login details for websites you frequent, you'll see that information here. Tap Edit to select multiple login items to delete, or swipe to the left on single items to delete individual listings.
Select a saved login to view the website, username and password.
To add a new account, you'll need to scroll all the way to the bottom of the list and tap Add Password. There you can enter the website, username and password. Right now, it doesn't appear that the app will suggest complex passwords for you, so be sure to come up with something on your own. A complex password or random characters is ideal, and since iOS 11 will fill it in for you in future, don't worry about making it easy to remember. Let iOS 11 do the work.
reading•iOS 11 means you never have to remember app passwords again
Nov 18•I went to buy an iPhone X. AT&T told me Galaxy Note 8 is better
Nov 18•HomePod delay clouds Apple’s smart speaker future
Nov 18•Apple Park opens its doors to outsiders
Nov 17•iPhone X is one of Time's 25 best inventions of 2017 (Apple Byte Extra Crunchy, Ep. 110)