On the pen-enabled Galaxy Note II, users have long been able to get more information about many onscreen items simply by hovering over them with the S Pen stylus. Of course, the Galaxy S4 doesn't come with an S Pen, but it allows you to get the same Air View functionality by holding your finger above the right area of the screen.
Using Air View, you can get larger blow-ups of thumbnails in the gallery, see preview thumbnails of upcoming scenes when you jog through a video or view the first couple sentences of an email in a pop-up window when you hover over a message in your inbox. You can even see thumbnail previews of the tabs in your Web browser by holding your finger just above the screen. No stylus required.