The reader or audience knows something the character does not, so the character's words and actions take on a meaning he does not perceive himself.
In the Aeneid
Such things, on the shield of Vulcan, a gift from his mother, he admires, and not knowing these things, he rejoices in the image, lifting to his shoulder the fame and the destinies of his descendants.
The narrator says that Aeneas rejoices in the image “not knowing these things”: he lifts to his shoulder the destiny of Rome without deciphering it. The reader, who recognizes each scene engraved on the shield, understands more than the hero does.