Plato “Allegory of the Cave”

Explain one of the following excerpts:

"And now, I said, let me show in a figure how far our nature is enlightened or unenlightened:--Behold! human beings living in an underground den, which has a mouth open toward the light and reaching all along the den; here they have been from their childhood, and have their legs and necks chained so that they cannot move, and can only see before them, being prevented by the chains from turning round their heads."