Short for transform, clipping, and lighting, T&L or TCL is a video technology that takes all the 3D information that used to be handled by the computer processor and gives it to the GPU. Having the GPU handle 3D information allows for a more complex 3D environment with a higher polygon count and improved lighting; while allowing the computer processor to handle other tasks.
Some computer games may require that users have a T&L compatible chipset. If you are unsure if your video card or motherboard video chipset is T&L compatible or what generation of T&L your computer has, check the documentation for your video card. If your video card or motherboard chipset is not T&L compatible, you need to install a new video card in the computer that is T&L compatible.