Plain English Meaning
A runway light system with three rows of lights that gives two correct approach angles to the same runway — a normal one for most aircraft and a slightly steeper one for aircraft whose pilots sit very high above the wheels.
Full Definition
A Visual Approach Slope Indicator system that uses three rows (bars) of light units instead of the standard two, providing two separate visual glide paths to the same runway. The lower glide path (typically about 3 degrees) is shown by the near and middle bars and is used by smaller aircraft. The upper glide path (typically about 3.25 degrees) is shown by the middle and far bars and is used by pilots of high-cockpit aircraft, such as widebody jets, so the wheels still cross the threshold at a safe height.