Plain English Meaning
A set of lights next to the runway that tells the pilot, by color, whether they are too high, too low, or on the right slope coming in to land.
Full Definition
A ground-based light system installed beside a runway that gives the pilot a visual indication of whether the aircraft is on, above, or below the correct descent path to the runway during approach. VGSI is a general category that includes systems such as VASI (Visual Approach Slope Indicator) and PAPI (Precision Approach Path Indicator).