The chains that prevent the prisoners from leaving the cave represent that they are trapped in ignorance, as the chains are stopping them from learning the truth.The shadows cast on the walls of the cave represent the superficial truth, which is the illusion that the prisoners see in the cave.
The allegory is probably related to Plato's theory of Forms, according to which the "Forms" (or "Ideas"), and not the material world known to us through sensation, possess the highest and most fundamental kind of reality.
Only knowledge of the Forms constitutes real knowledge or what Socrates considers "the good".
The artists use light and shadows to teach the dominant doctrines of a time and place. This is not some easy task, and only a true philosopher, with decades of preparation, would be able to leave the cave, up the steep incline. The light would hurt his eyes and make it difficult for him to see the objects casting the shadows.
Most humans will live at the bottom of the cave, and a small few will be the major artists that project the shadows with the use of human made light. If he were told that what he is seeing is real instead of the other version of reality he sees on the wall, he would not believe it.
The allegory contains many forms of symbolism used to instruct the reader in the nature of perception. It also represents ignorance, as those in the cave live accepting what they see at face value.
Ignorance is further represented by the darkness that engulfs them because they cannot know the true objects that form the shadows, leading them to believe the shadows are the true forms of the objects.
Plato has Socrates describe a group of people who have lived chained to the wall of a cave all of their lives, facing a blank wall.
The people watch shadows projected on the wall from objects passing in front of a fire behind them, and give names to these shadows. Socrates explains how the philosopher is like a prisoner who is freed from the cave and comes to understand that the shadows on the wall are not reality at all, for he can perceive the true form of reality rather than the manufactured reality that is the shadows seen by the prisoners.
Socrates suggests that the shadows are 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 things outside the cave which they do not see (514b-515a).
The fire, or human made light, and the puppets, used to make shadows, are done by the artists.