Moonlight is a small white/blue light or a series of them above the tank. Used to be that people used typical incandescent 20W blue bulbs. Now its more common to find rope lights or single blue/white LEDs as moonlights. Basically its there for ease of nighttime viewing, although some believe its helpful for the corals (if you want them to spawn in a closed system).
Have a search at the sponsors page for moonlights, I'm sure you'll find many examples.