The Allegory of the Cave
"Plato recognized that the picture of the Divided Line may be difficult for many of us to understand. Although it accurately represents the different levels of reality and corresponding degrees of knowledge, there is a sense in which one cannot appreciate its full significance without first having achieved the highest level. So, for the benefit of those of us who are still learning but would like to grasp what he is talking about, Plato offered a simpler story in which each of the same structural components appears in a way that we can all comprehend at our own level. This is the Allegory of the Cave.
Suppose that there is a group of human beings who have lived their entire lives trapped in a subterranean chamber lit by a large fire behind them. Chained in place, these cave-dwellers can see nothing but shadows (of their own bodies and of other things) projected on a flat wall in front of them. Some of these people will be content to do no more than notice the play of light and shadow, while the more clever among them will become highly skilled observers of the patterns that most regularly occur. In both cases, however, they cannot truly comprehend what they see, since they are prevented from grasping its true source and nature. (Republic 514a)
Now suppose that one of these human beings manages to break the chains, climb through the torturous passage to the surface, and escape the cave. With eyes accustomed only to the dim light of the former habitation, this individual will at first be blinded by the brightness of the surface world, able to look only upon the shadows and reflections of the real world. But after some time and effort, the former cave-dweller will become able to appreciate the full variety of the newly-discovered world, looking at trees, mountains, and (eventually) the sun itself.
Finally, suppose that this escapee returns to the cave, trying to persuade its inhabitants that there is another, better, more real world than the one in which they have so long been content to dwell. They are unlikely to be impressed by the pleas of this extraordinary individual, Plato noted, especially since their former companion, having travelled to the bright surface world, is now inept and clumsy in the dim realm of the cave. Nevertheless, it would have been in the best interest of these residents of the cave to entrust their lives to the one enlightened member of their company, whose acquaintance with other things is a unique qualification for genuine knowledge.
Plato seriously intended this allegory as a representation of the state of ordinary human existence. We, like the people raised in a cave, are trapped in a world of impermanence and partiality, the realm of sensible objects. Entranced by the particular and immediate experiences these things provide, we are unlikely to appreciate the declarations of philosophers, the few among us who, like the escapee, have made the effort to achieve eternal knowledge of the permanent forms. But, like them, it would serve us best if we were to follow this guidance, discipline our own minds, and seek an accurate understanding of the highest objects of human contemplation."
Source: Philosophy pages.