Plain English Meaning
A long line of bright white lights stretching out from the runway, with one set of lights flashing in sequence toward the runway like a chasing arrow. It's designed to help a pilot find the runway when breaking out of clouds on an instrument approach.
Full Definition
A high-intensity approach lighting system installed at the approach end of a Category I precision instrument runway. It consists of a 2,400-foot centerline of steady white lights extending outward from the runway threshold, a crossbar of steady white lights at 1,000 feet from the threshold, a green threshold bar, red terminating bars near the threshold, and a series of high-intensity sequenced flashing white lights (a '') that fire in rapid succession from the end of the toward the runway, leading 's eye to the threshold during a - .