Plain English Meaning
If your radio stops working, the tower can shine a colored light at your aircraft to tell you what to do. Different colors and flashing patterns mean different things, like 'cleared to land' or 'return to airport.'
Full Definition
A standardized set of colored light beams (steady or flashing green, red, or white) directed from the control tower at an aircraft to convey clearances and instructions when normal two-way radio communication is not available. Each color and pattern carries a specific meaning that differs depending on whether the aircraft is in flight or on the ground.