PAPI, or Precision Approach Path Indicator, is a visual aid used in aviation to help pilots land safely. It consists of a series of lights that indicate whether an aircraft is on the correct glide path during its approach to a runway. The lights change color based on the aircraft's altitude relative to the ideal approach angle, guiding pilots to adjust their descent. Typically, PAPI systems have four lights positioned to the side of the runway. When two white and two red lights are visible, the aircraft is on the correct glide path. If more white lights are seen, the aircraft is too high, while more red lights indicate it is too low.