Thanks to Face ID, the iPhone X knows when its owner is looking at it, and can hide the content of your notifications until you do so. Now, if somebody else picks up your iPhone X and takes a peek at your incoming alerts, it will only see a list of the apps that have notifications for you. The content of the alerts remains hidden until you look at the screen, and Face ID expands the boxes to show you your messages.
The twist is that you can already do something very similar with Touch ID, just by changing one setting.