When your iPhone is locked, iOS will try to mask the contents of your Lock Screen’s incoming notification banners until you authenticate yourself so that onlookers who shouldn’t be snooping on your notifications won’t be able to in the first place.
This is a brilliant privacy feature on Apple’s part, and such a feature may also have uses when your iPhone is unlocked, such as when showing a friend or co-worker your iPhone’s screen to look at a photo or video and an unexpected notification banner pops up.