Allegory of the Cave
Plato begins by asking Glaucon to imagine a cave where people have been imprisoned from childhood. These prisoners are chained so that their legs and necks are fixed, forcing them to gaze at the wall in front of them and not look around at the cave, each other, or themselves (514a–b). Behind the prisoners is a fire, and between the fire and the prisoners is a raised walkway with a low wall, behind which people walk carrying objects or puppets “of men and other living things” (514b). The people walk behind the wall so their bodies do not cast shadows for the prisoners to see, but the objects they carry do (“just as puppet showmen have screens in front of them at which they work their puppets” (514a)). The prisoners cannot see any of this behind them and are only able to see the shadows cast upon the cave wall in front of them. The sounds of the people talking echo off the shadowed wall, and the prisoners falsely believe these sounds come from the shadows (514c).
Socrates suggests that the shadows constitute reality for the prisoners because they have never seen anything else; they do not realize that what they see are shadows of objects in front of a fire, much less that these objects are inspired by real living things outside the cave (514b-515a).
Departure from the cave
Plato then supposes that one prisoner is freed, being forced to turn and see the fire. The light would hurt his eyes and make it hard for him to see the objects that are casting the shadows. If he is told that what he saw before was not real but instead that the objects he is now struggling to see are, he would not believe it. In his pain, Plato continues, the freed prisoner would turn away and run back to what he can see and is accustomed to, that is the shadows of the carried objects. He writes “…it would hurt his eyes, and he would escape by turning away to the things which he was able to look at, and these he would believe to be clearer than what was being shown to him.” 
Plato continues: “suppose…that someone should drag him…by force, up the rough ascent, the steep way up, and never stop until he could drag him out into the light of the sun.”  The prisoner would be angry and in pain, and this would only worsen when the radiant light of the sun overwhelms his eyes and blinds him. The sunlight is representative of the new reality and knowledge that the freed prisoner is experiencing.
Slowly, his eyes adjust to the light of the sun. First he can only see shadows. Gradually he can see the reflections of people and things in water and then later see the people and things themselves. Eventually he is able to look at the stars and moon at night until finally he can look upon the sun itself (516a). Only after he can look straight at the sun “is he able to reason about it” and what it is (516b). (See also Plato’s Analogy of the Sun, which occurs near the end of The Republic, Book VI.)
Return to the cave
Plato continues, saying that the freed prisoner would think that the real world was superior to the world he experienced in the cave; “he would bless himself for the change, and pity [the other prisoners]” and would want to bring his fellow cave dwellers out of the cave and into the sunlight (516c).
The returning prisoner, whose eyes have become acclimated to the light of the sun, would be blind when he re-enters the cave, just as he was when he was first exposed to the sun (516e). The prisoners, according to Socrates, would infer from the returning man’s blindness that the journey out of the cave had harmed him and that they should not undertake a similar journey. Socrates concludes that the prisoners, if they were able, would therefore reach out and kill anyone who attempted to drag them out of the cave (517a).
Read more at : https://en.wikipedia.org/wiki/Allegory_of_the_Cave