The visual approach slope indicator (VASI) is a system of lights on the side of an airport runway threshold that provides visual descent guidance information during approach. Specifically, the indicator shows an aircraft if it is too high or too low to hit the correct landing spot on a runway.
By far, the most popular form of VASI in Second Life is a version modelled on the real-life tri-color VASI. A simple system, it makes use of three colored lights which indicate whether the pilot is too high (white), too low (red), or on the correct glidepath (green).
Diagram of a tri-color visual approach slope indicator (VASI).