To me, representation in literature is literature that takes a more allegorical or symbolic form than other, more concrete, literature. It allows for a more fluid style of storytelling, where certain things with common characteristics can be linked when they might otherwise have been. It also allows for a much more ambiguous meaning and more room for interpretation.
An excellent example off of the top of my head is Dante’s The Divine Comedy. While the themes the author was trying to promote might have been clear, the way he told the story allowed for archetypal representations in a novel. He wasn’t telling a story in the traditional sense, in that there is a conflict that a hero must overcome; he was telling the story of a man who walked through the different types of afterlife. But the focus wasn’t on the story; it was on what the story was telling its audience, which is where the allegorical elements come into play.