Imagine prisoners who have been chained since childhood deep inside a cave. Not only are their limbs immobilized by the chains, their heads are chained as well so that their eyes are fixed on a wall.
Behind the prisoners is an enormous fire, and between the fire and the prisoners there is a raised walkway, along which shapes of various animals, plants, and other things are carried. The shapes cast shadows on the wall, which occupy the prisoners' attention. Also, when one of the shape-carriers speaks, an echo against the wall causes the prisoners to believe that the words come from the shadows.
The prisoners engage in what appears to us to be a game - naming the shapes as they come by. This, however, is the only reality that they know, even though they are seeing merely shadows of images.
Suppose a prisoner is released and compelled to stand up and turn around.
His eyes will be blinded by the firelight, and the shapes passing will appear less real than their shadows.
Similarly, if he is dragged up out of the cave into the sunlight, his eyes will be so blinded that he will not be able to see anything.
At first, he will be able to see darker shapes such as shadows, and only later brighter and brighter objects.
The last object he would be able to see is the sun, which, in time, he would learn to see as that object which provides the seasons and the courses of the year, presides over all things in the visible region, and is in some way the cause of all these things that he has seen.
(The Republic bk. VII, 516b-c; trans. Paul Shorey)
Plato's metaphor of the sun:
Once thus enlightened, so to speak, the freed prisoner would no doubt want to return to the cave to free "his fellow bondsmen." The problem however is that they would not want to be freed: descending back into the cave would require that the freed prisoner's eyes adjust again, and for a time, he would be inferior at the ludicrous process of identifying shapes on the wall. This would make his fellow prisoners murderous toward anyone who attempted to free them.
(The Republic Bk. VI; trans. Paul Shorey)