The Fresnel lens optical landing system provides guidance for correctly landing on an aircraft carrier.
The lens is located on the side of the runway so that it can be seen by the pilots throughout the entire landing process.

The optical landing system consists of a horizontal bar of green lights and a vertical bar of red lights on both sides of the "meatball". The "meatball" is the centerpiece that consists of five amber colored lenses.
Certain lenses will light up one at a time depending on the angle the plane is in relation to the "meatball." This causes the center light to appear to be moving up and down in relation to the horizontal green bars on the sides.
In order to safely land, the pilot tries to keep the center amber lens horizontal with the green bar throughout process .
If the pilot gets too low, the amber light will turn red indicating that the aircraft is dangerously low and risks hitting the back end of the aircraft carrier.
The red lights around the green horizontal bars will be flashing if the carrier is not able to receive the aircraft, and so the jet must keep circling or find another place to land.

From Illumin Volume 9: Issue ii, USC School of Engineering.