A sheen is a quality of things that are shiny, usually with reflected light. Anything that shines has a sheen, like satin in candlelight or a beautiful head of hair.
Shakespeare loved using words in fresh new ways, and he did that with the adjective for sheen (for “beautiful, bright”) in "Hamlet." He used sheen as a noun, and it’s been that way ever since. A sheen is a luster that happens because of reflected light. A well-polished table has a sheen, as does a car that's just been washed. Shined shoes have a sheen. If it’s shiny and beautiful, it has a sheen.