Triple Buffering: In normal rendering, the GPU uses a technique known as Double Buffering to store two graphics frames in the Video RAM - one frame of graphics which the GPU is currently working on, and one frame which has been completed and is sent to the display. When Vertical Synchronization (See the Vertical Sync setting further below) is enabled in a game, your GPU will become synchronized to your monitor's refresh rate. In this environment, if double buffering is used, and your framerate falls below the refresh rate, the GPU may fill both frame buffers and become temporarily idle while it waits for the display's next refresh cycle so it can free up one of the buffers. The end result is a reduction in performance, sometimes by quite a significant amount (e.g. 50%), as the GPU is underutilized. Triple Buffering adds a third frame buffer to the process, specifically aimed at preventing the GPU from becoming idle due to waiting for the display to refresh, thus maintaining maximum performance when VSync is enabled. Triple Buffering is covered in more detail on this page of the Gamer's Graphics & Display Settings Guide.
The available options for this setting are Off and On. However, although not clearly indicated by Nvidia in the NVCP, this setting only applies to OpenGL games. Since the vast majority of games are based on DirectX, this setting won't have any impact on them. An Nvidia Engineer confirms this in this article by noting that there is no driver override functionality for Triple Buffering under DirectX. This has been done for compatibility purposes, since DirectX games use internally controlled frame buffering, and forcing Triple Buffering via the graphics driver can break this internal buffer management. If you wish to use Triple Buffering in a DirectX game, check the in-game settings for buffer-related options. Many games don't have any such options, in which case, to force Triple Buffering in DirectX games, you can use the D3DOverrider utility covered under the Advanced Tweaking section later in the guide.
Some important things to keep in mind regarding Triple Buffering: