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 exceed the polygon limit of retail FreeSpace (somewhere around 800 polies). 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