The allegory of the cave, in Plato's Republic, is described by Plato's master, Socrates, in dialogue with a disciple. It illustrates Plato's Theory of Forms, the idea that the human mind is only capable of grasping a limited representation of reality, not seeing reality itself.
Socrates asks his listeners to imagine a group of human beings chained to a wall in a cave and also facing a blank wall. Behind the prisoners, objects pass before a fire projecting shadows on the wall which the prisoners face, but they are unable to turn their heads to see these objects, and accept the shadows as reality. When they discuss the shadows, the believe that the words they use accurately describe the objects they are unable to see.
Imagine, then, that one prisoner is able to break free from their chains and make a very painful ascent upwards out of the cave and into the sunlight. After the former prisoner goes through the difficult process of becoming accustomed to the bright light of the sun, this person would finally gain a truer perception of the real world than those still in the cave.
Yet, if this enlightened former prisoner returns to the darkness of the cave, and reveals to the prisoners, who continue to believe in the accuracy of their perceptions, that they are deluded in this belief, what would happen?
SOCRATES: And if they [the prisoners) can take this person (the enlightened prisoner) to free them from their chains and lead them up, if they could kill him, would they actually kill him?
GLAUCON: They certainly will.
The allegory can be interpreted epistemologically, to mean that only through the enlightenment of philosophical training can people become aware that they are unable to perceive reality itself, but only a "shadow" of reality, given the limits of the senses. It can also be interpreted politically to mean that those who have been enlightened may be in danger when they try to introduce their ideas to those still dwelling in the world of shadows. Indeed, this was the fate of Socrates.