These are programs that emulates the defunct 3Dfx's obsolete glide technology in modern video cards. Glide is needed to play King's Quest 8 to the fullest quality it's capable of. With glide enabled one can push the game into 800X600 resolution with all the special effects turned on. The two most compatible glide wrappers for KQ8 are dgVoodoo2 (DirectX 11) and Zeckensack's glide wrapper (DirectX 9).
The current version of NGlide has apparently fixed the problems that previous versions had with the game, and will work properly now as well.