A cave lit by a fire with the shadows of objects shown on its furthest wall.
Chained prisoners inside the cave have only every perceived this experience of the world, and believe their perceptions to be reality.
These prisoners are ignorant to the outside world, and therefore unwilling to accept that any other reality exists.
When one of the prisoners is freed, and allowed to experience the outside world, he is at first pained by the bright lights and can only comprehend shadows an reflections. As his vision becomes stronger and his knowledge expands, he is able to see the world as it truly is.
Because of their lack of understanding of the outside world, the other prisoners do not comprehend the freed prisoner's new wisdom, and ridicule him when he tries to share his knowledge. Because of their firm belief that the world they have grown to know is most true form of reality, they are incapable of grasping the context of this new understanding.
The allegory illustrates the process and struggle of becoming enlightened. At first, understanding the truth can be painful and frightening because it can undermine one's entire established understanding of reality. When this newly enlightened person attempts to share what they now know, those who have not had the ability to understand the truth cannot relate to this person, and because of their ignorance, believe their narrow perceptions to be more true.