Plato's Allegory of the Cave is Book VII in his much larger work, The Republic. The Allegory of the Cave is a conversation between Socrates and Glaucon, the former telling us what he believes about reality and how we have a false interpretation of reality. It is an extended metaphor full of symbolism. All of the objects in the story mean something, and can be applied to the world we live in.
Socrates starts his story by describing an image which is pointedly "strange", according to Glaucon. He tells about prisoners who have been chained since childhood in a cave. The prisoners have always faced one wall and have only seen shadows. They have not ever seen themselves or their neighbors. Behind them is a fire and in front of the fire is a roadway, on which people pass by. The prisoners can only hear the voices behind them.
The cave is the "sight world", where both the sight of truth and the sight of untruth exists. It is not a physical world that we can see, like earth, but a spiritual one which can only be seen by the soul. The fire represents truth, reality, and the ultimate gain of knowledge.
The prisoners are humans, like you and me. Their chains represent the restraints that society has put on our sight. We know this because ever since childhood, we have been trained by society to limit our perceptions. We see whatever society has taught us to see and only imagine reality. The prisoners only see the shadows on their wall which they believe to be reality. They are trapped in the part of the cave where untruth, or unreality exists.
The voices are those of the instructors, those who show truth. The instructors are the ones who pass by on the roadway. Most of the prisoners disregard the voices perceiving them to be from untruth- the shadows. Very few prisoners, like the freed prisoner, see that it is not untruth, but something else. The voice calls to one of the prisoners, telling him that all his life he has not known reality, but was given a lie.