The allegory contains many forms of symbolism used to describe the state of the world.
The cave is a symbol of the world and the prisoners are those who inhabit the world.
The chains that prevent the prisoners from leaving the cave represent ignorance, meaning they interfere with the prisoners seeing the truth. The shadows cast on the walls of the cave represent what people see in the present world.
Last, the freed prisoner represents those in society who see the physical world for the illusion that it is.