The Precision Approach Path Indicator (PAPI) is a light system positioned beside the runway that consists of two or four boxes of lights that provide a visual indication of an airplane's position on the glidepath for the associated runway. The FAA standard for the PAPI is the same as the ICAO's standard Visual Glide Slope Indicator.
Each box of lights is equipped with an optical apparatus that horizontally splits light output into two phases: red and white. Depending on the viewing (and thus the descent) angle, a pilot will see either the red or the white portion.
A normal glidepath is indicated by an even split in red and white lights. If an airplane is beneath the glidepath, more red lights visible to the pilots; if an airplane is above the glidepath, more white lights are visible.
During aircraft descent, this system, along with other airport lights, may be activated by the pilot by keying the airplane microphone with the airplane's communication radio tuned to the CTAF.