The Wii U's whole sales pitch is that its primary controller, the Wii U Gamepad, is also a touchscreen "tablet." But it stops working entirely if it gets too far from the console, so don't get any big ideas.
It means two big things: First, that games can get touchscreen controls — games like "New Super Marios Bros. U" for the Wii U let one player touch the screen to freeze enemies or hold moving platforms in place.
Second, it means that you can actually play many Wii U games on the smaller screen by itself, no TV required. It's a handy thing if you live somewhere where television time is hard to come by.