When quick access and confidence in your content is important, 3D Touch gives you the ability to instantly preview what's most important.
Press on the item of content you wish to peek at with more pressure and for longer than a tap.
The rest of the screen will blur and the item of content will appear highlighted. This is called Peeking.
If the peek is enough, let go to dismiss it.
If you want to open the document, press deeply on the screen again and the content item will appear in a new window which will allow you to scroll and browse. This is called Popping.