VASI, or Visual Approach Slope Indicator, is a visual aid used in aviation to help pilots maintain the correct approach angle when landing. It consists of a series of lights that indicate whether the aircraft is above, below, or on the ideal glide path. This system enhances safety by providing real-time feedback to pilots during the landing phase. The VASI system typically features two or four light units positioned near the runway. When the lights are green, the aircraft is on the correct glide path; red lights indicate that the aircraft is too low. This simple yet effective tool is crucial for ensuring safe landings, especially in low-visibility conditions.