Off-TV Play is an optional feature for games developed for the Wii U. Specifically, games normally played using the TV as the primary video source can instead use the Wii U GamePad screen to display the game's action. This function makes it possible to play games on the Wii U while leaving the television free for other activities. The GamePad screen displays the game video without additional latency and supports 60 fps streaming.
Though Off-TV Play is supported at the system level, it is up to the developers as to whether or not it will be an option for specific games. In particular, games designed to require use of a dual-screen TV/GamePad display would not benefit from an Off-TV Play mode. How the mode is enabled also varies from game to game; some titles allow the mode to be enabled from a menu option, while for other games, it can be enabled or disabled via a button press at any time.
Remote Play is a similar feature available on Sony's PlayStation 3 and PlayStation 4, but requires the use of a PSP or Vita respectively.