Short for Transform and Lighting, T&L 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. This enables for a more complex 3D environment by adding a higher polygon count and improving the lighting; at the same time it allows 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 will need to install a new video card in the computer that is T&L compatible.
Also see: Video definitions