There was a scene from the film Troy, after Achilles had killed a giant warrior who was clearly invincible (until he met Achilles, anyway) when a man said to him, "I would never have dared to fight him." And Achilles replied, "Then history will never remember you."
To Dream The Impossible Dream encapsulates that idea. It means, "Don't sell yourself short. Don't underestimate yourself. Dare to try."