As per usual with our man Plato, he doesn’t have the courage of his own convictions, and rather than just come out and describe one of the greatest metaphors in the history of Western philosophy, he embeds it in a dialogue between Socrates and Glaucon (Plato’s brother).
The allegory is simple enough: picture a group of people chained in a cave. They face a blank wall, and there is a light source behind them – so that they only things they ever see are the shadows cast by whatever moves between them and the light. Inevitably, they come to ascribe meaning to the shadows they see, and to believe that they are all there is of reality…
…the philosopher, naturally, is a freed prisoner in this analogy: it is his role to describe the true reality to the other prisoners. If you’ve any familiarity with Plato’s work, you’ll recognise here his familiar concepts of Platonic forms (i.e. true and ideal forms) opposed to mundane forms (i.e. the ones in this world).