Imagine you are in an underground cave.
The entrance and exit leads upward to daylight. You, among many other prisoners have been chained to the ground and by the head in the cave every since you were a child. You can only see what is on the wall in front of you as your head is fixed in that position. Fire is burning behind the you. Between the burning, hot fire and yourself, people walk, talk and carry objects. You perceive only shadows of the people and things passing on the walkway; the prisoners hear echoes of the talk coming from the shadows. Thus, you and the fellow prisoners perceive the shadows and echoes as reality.
After years in the cave, you and only you are released among the prisoners. You turn around. What do you see? Are you frightened? Pained by new physical movement? Dazzled by the fire? Unable at first to see, you are told that the people and things you now perceive are more real than the shadows you've seen for all your life. Will you believe it? Would you want to return to your old perceptions of the shadows as reality? Dragged out of the cave, you are blinded by the sun but you gradually see the stars and the moon; then be able to see shadows in the daylight thrown by the sun; then objects in the full light of day. The sun makes this new perception possible. If you were taken back to the cave, the world you once knew, do you think you will be able to function well in the world of shadows? No.
This is the Allegory of the Cave.
Once one of the prisoner embraces the sun, the freed prisoner would believe that the real world was far more 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"
As the free prisoner returns, he would not be accustomed to the dark caves which is in fact very similar to when he was first step foot into the sun. The prisoners would assume that his journey to the outside has harmed his very being and that they should not follow in the freed prisoners footsteps.
To conclude, the prisoners would rather stay in the cave than explore the outside world. They persevere to stay in the cave so much that they would go to the extent of killing or harming those who attempt to drag them out (much like what happened to Socrates).
The prisoners in the cave are classified as ignorant people. Once they are freed and walk nearer and nearer to the entrance/exit, the higher their intellect is (the closer you get to the entrance/exit and the outside, the smarter you are).
Plato believes that all the stages of intellect are intertwine.
When the prisoner returns to the Cave in order to lead his fellows to the light of understanding, they may be so dismayed at their having been wrenched from their comfortable state of ignorance that they may want to kill him — a likely allusion to the death of Socrates. And the allusion is amplified: If the first prisoner, now enlightened by his contemplation of Justice itself, were to be hauled into a courtroom and faced with the unenlightened quibbles of lawyers trained in sophistry, he probably would not be able to defend himself.
Belief vs Reality
As suggested by Socrates during his discussion with Glaucon, Plato's brother, the shadows amount to reality for the prisoners as they have never seen anything else. The prisoners did not realize the fact that those objects they see on the wall were shadows of the objects in front of the fire. Moreover, they did not know that the objects were things that were from outside of the cave.