The number 666 is used to signify the devil, the antichrist, or evil in general. It’s not be confused with another, unrelated 666, used by Chinese gamers to commend skillful play.
Use in The Phantom Of The Opera
At the beginning of the movie, the auctioneer brings out a chandelier in pieces labeled 666, and remarks that it is "the very chandelier in the famous disaster." He is referring to how the phantom would later in the movie crash the chandelier. 666 is a reference to the phantom, and the evil that he has done.