It's a meme from the game Elder Scrolls V: Skyrim. The main character, it turns out, is some sort of mythical being known as a Dragonborn, who can speak the language of dragons. That language has various effects on the main character and on the world, like giving you bursts of superspeed and letting you telekinetically push things away from you. You learn that last one in levels, so when you yell, 'Fus!" it's just sort of a little shove against whatever's right in front of you. When you get to "Fus Ro!" it's stronger, but when you learn the whole thing, it's "Fus Ro Dah!" which pushes anyone in front of you so hard they end up flying off into the distance. And then it turned into a YouTube meme:
