PAPI (Precision Approach Path Indicator)

It is a system of lights placed on the sides of the runway for landing/takeoff. It consists of light boxes that provide a visual indication of an aircraft's position relative to the approach path associated with a particular landing/takeoff runway.

The approach paths vary depending on the conditions and situation of the runway, with a common value being 3º. 

The system consists of 4 lamps that emit a light beam of two segments, each in a color: white (upper) and red (lower).

The defined combinations are:

Four white lights: the aircraft is much above the path indicated by the indicator.

Three white lights and one red: the aircraft is slightly above the path.

Two white lights and two red lights: the aircraft is on the correct approach path.

Three red lights and one white: the aircraft is slightly low.

Four red lights: the aircraft is flying very low and must climb.