This quote is taken from William Shakespeare's Hamlet. In Act 2, Scene 2, Guildenstern says, "The very substance of the ambitious is merely the shadow of a dream." which means the reality in which all ambitious people live is intangible, much like a shadow, a fantasy or a dream that is yet to be materialized. Shadows and dreams both are intangible, shadows require light and objects in order to be cast. In the same scene, the dialog between Hamlet, Guildenstern and Rosencrantz ends where Hamlet and Rosencrantz concur to this idea, and Hamlet says, "But a dream itself is just a shadow" and Rosencrantz says, "Exactly. In fact, I consider ambition to be so light and airy that it’s only the shadow of a shadow". Gathering from the context this quote can simply be translated to, people of ambition have dreams that are not materialized yet, but the fact that they have them, let's them live those dreams. The dialog ends with Hamlet saying, "Then I guess beggars are the ones with bodies, while ambitious kings and heroes are just the shadows of beggars. Should we go inside? I seem to be losing my mind a bit."