They have been there since childhood, with their necks and legs fettered, so that they are fixed in the same place, able to see only in front of them, because their fetter prevents them from turning their heads around.
Light is provided by a fire burning far above and behind them. Between the prisoners and the fire, there is an elevated road stretching. Imagine that along this road a low wall has been built--like the screen in front of people that is provided by puppeteers, and above which they show their puppets.
Glaucon: I am imagining it.
Socrates: Also imagine that there are people alongside the wall carrying mutlifarious artifacts that project above it--statues of people and other animals, made of stone, wood, and every material. And as you would expect, some of the carriers are talking, and some are silent.
Glaucon: It is a strange image you are describing, and strange prisoners.
[text by Plato (The Republic, Book VII, 514a-c); illustration by Anora Johnson of Champaign, Illinois. Please do not reproduce the images without artist's consent.]