Note to Hamlet, 2.2.263-264: "Then are our beggars bodies, and our monarchs and outstretched heroes the beggars' shadows"

The beggars are "bodies," solid substances, in contrast to "shadows," because the beggars have no ambition, no dreams of being something more. On the other hand, the "monarchs and outstretched heroes" are only what they are because the beggars think so. A person who is "outstretched," who has far-reaching ambitions, is nothing unless the beggars think he is indeed a hero. So that hero is actually a shadow, a creation of the dreams of common people.

To Hamlet, this is another example of the idea that "there is nothing either good or bad, but thinking makes it so."