Luster is literally the gleam of gold. It's a shining effect, and used as a metaphor it describes an image of something of value, real or imagined. The fascination of gold's luster is the analogy. In technical terms, luster is also another word for a shining effect, usually a superlative rather than an actual description. It can also be a metaphor for the image of an idea or a reference to a personal image.