IPhone 6s what is this? A glitch or a feature?

From time to time I'll be browsing on Safari or on my messaging app and this happens. Where the app I am on just moves down and there is a black space covering up the upper part of the phone. Is this some sort of feature or glitch? If it's a feature, what's it for?
This is easily fixable by pressing the home button.

Read more: IPhone 6s what is this? A glitch or a feature?