In iOS 11, your iPhone gives you much more control over notifications. You can designate certain apps as “sensitive”, so that it hides the content of notifications while your phone is locked, only letting you see the full preview when you use Touch ID or Face ID to unlock your iPhone. This works in every single app on your phone, unlike in iOS 10 and before.
Previously, only Mail and Messages allowed for this—third party apps had to build in their own similar features if you wanted to hide the notification content from the lock screen. But now, this is built into iOS 11 for all apps. When you designate an app as “sensitive”, it won’t show the contents of your notification—for example, the entire text message or email subject—instead it’ll just say “Notification”.