Plato is saying that the things we see are merely shadows of real things. Real things are represented by models of the things that are being carried in front of the fire, and since we are "chained" so that we cannot see anything but "shadows, this is what we perceive as real. However, when one is "unchained" and led up into the sunlight, this represents what Plato calls "the forms." This means that everything is modeled after an "ideal" form of what it is, for instance, a dog on earth is modeled after the "form" of dog, but since earth is flawed, the dog has flaws. Plato says that when we come "outside," which means that we open our minds to the ideas of the "forms" we can begin to comprehend the universe. But until we are "unchained" we will continue to believe that the "shadows" are the real thing.