This is a very good question and one that is important as it hits on a central point in Plato's philosophy.
Here is some background information. The allegory of the cave is found in Plato's Republic. (531-534) The basic content of the allegory is as follows. There are people chained in a cave and they are turned to face a wall. They live their lives in bondage. The only things that they can see are the shadows that are cast on the wall, which are projected from a fire. In time, these people believe that what they see are realities, when in fact they are merely shadows.
Plato's point is that through philosophy, people are able to see through the shadows and perceive reality for what it really is. All of this is directly tied to Plato's theory of the forms. Plato believes that through rigorous philosophy people can see beyond the "shadows" of this world and see the true eternal forms. So, people can know what goodness, justice, and the like, are in their essence by knowing the true form of goodness and justice. Without philosophy, according to Plato, people are "chained to a wall" and they are only able to see "shadows."
Enlightenment, but also knowledge of the "forms." The shadows represent things in reality that appear to be real things in themselves but are actually reflective of things that exist in another realm of which they take part. That sounds a little esoteric to me.