The allegory of
Plato's cave found in
The Republic is about our own knowledge of the world and how we perceive it.
In this cave there are prisoners chained so that they are facing a wall. Behind them, a great fire (the philosopher Heraclitus beloved that fire was the primary form of reality - see Stasis, Change and Atoms in Ancient Greek Philosophy for more on this subject). Between the prisoners and the fire a puppet show is happening. The prisoners see the shadows on the walls and this is their reality.
A person freed from his chains and looking towards the light will be blinded and not even able to see what realities he did know before. Being told that what he just saw will confuse him, after all he has spent a lifetime watching shadows. Many of those people will choose instead to go back to the painless knowledge of the shadows rather than the light.
Suppose that one of these prisoners is dragged up and held fast before the sun itself. He will be near blind until his eyes adjust, but when they do he will be dazzled and not able to see anything that resembled what he knew as reality before. In time, the prisoner will be able to see all the world around him. Through this knowledge he will become aware of his place in the world.
Meanwhile, back in the cave, prisoners give honor to those among themselves who noticed the passing shadows first and their happenings. This prisoner, who now sees the light and once respected those who remarked on shadows would have lost much of that respect. In trying to explain what he has seen to those back in the cave, after his eyes have adjusted to the sun, he would seem foolish to those in the cave. However, the prisoner would know what is right, even if all those around him disbelieve it.
Plato is not only talking of the realm of
forms, but is critiquing his peers and those before him, claiming they only see
shadows of
reality and talk about them as if they were real.
The cave is what we see, and the fire is the sun. What we see is light of the sun reflecting off of things. To talk about these things as if they were real and make conjectures about them, while they may be accurate do not get at the heart of the matter - the Form
A person who is set free, and goes out to see the real world, and what lies behind the shadows of light that we see. Having been enlightened to the nature of reality, it is not easy to explain this to others.