Refer to figure.
PAPI stands for Precision Approach Path Indicator. PAPI lights are a visual aid used to assist pilots during the approach phase of landing at an airport. These lights are typically installed on the side of the runway and provide guidance to pilots to help them maintain the correct glide path for a safe landing.

PAPI lights consist of a row of lights arranged in a horizontal bar. In most cases, there are four lights in a PAPI system. The lights are typically white and red, with two white lights on the left side and two red lights on the right side when viewed by an approaching aircraft.

Too High: If an aircraft sees more white lights than red lights, it indicates that the aircraft is too high relative to the ideal glide path. In this case, the pilot should adjust the aircraft's descent rate to descend and bring the lights into the correct alignment.

Too Low: Conversely, if an aircraft sees more red lights than white lights, it indicates that the aircraft is too low relative to the ideal glide path. The pilot should increase the aircraft's descent rate to ascend and align the lights correctly.

Correct glide path: The pilot will see an equal number of white and red lights. This indicates that the aircraft is at the correct altitude for a safe landing.

PAPI lights provide a simple and effective visual reference for pilots, allowing them to make precise adjustments to their approach angle and altitude during the landing phase, ultimately enhancing safety and accuracy during landing operations.