Hardware Transformation and Lighting
Hardware Transformation and Lighting, or HTL as it is commonly abbreviated, is a technique where certain calculations involved in the rendering (drawing) of the screen, such as lighting, are handled by your on-board video card (hardware) instead of being processed by your CPU (sometimes considered software rendering).
It is the feature in FreeSpace Open that allows users to use high-polygon models in game. For most users, this also speeds up gameplay by taking much of the load off of the CPU. Thus the slowdown the higher-quality models would normally cause becomes inconspicuous.
See also: .DDS Files