What did Plato mean by prisoners watching shadows in a cave?

Plato meant that most of us may be taking copies for the real thing, and that learning to see past them is hard, painful and unpopular. In Book VII of the Republic, he has his teacher Socrates describe people chained in a cave since childhood, able to look only at the wall in front of them. Behind them burns a fire, and along a low wall people carry objects and puppets. The prisoners see only the shadows and hear the echoes, and since they have never seen anything else, those shadows are their reality.

Now free one prisoner. The firelight hurts his eyes, and he wants to run back to the familiar shadows. Dragged outside, he adjusts slowly: first shadows, then reflections in water, then the things themselves, then the stars, and at last the sun. For Plato, this ascent mirrors education, and the world outside stands for his theory of Forms, the idea that what we reach through reason is more real than what our senses show us.

The twist is the return. Back in the dark, the freed man is blinded, and the others conclude the trip outside ruined his eyes. If they could, Socrates says, they would kill anyone who tried to drag them out. Scholars still argue over whether the story is mostly about knowledge or about politics, and films like The Truman Show and The Matrix keep retelling it.