Plain English Meaning
A small light beside the runway that tells you if you are coming in too high, too low, or just right. The color and whether it flashes tells you which one.
Full Definition
A pulsating Visual Approach Slope Indicator is a single-light-source glidepath aid that shows the pilot whether the aircraft is on, above, or below the proper approach path by varying the color and pulse rate of the light. A steady white light indicates on glidepath, a pulsating white light indicates above glidepath, a steady red light indicates slightly below glidepath, and a pulsating red light indicates well below glidepath.