What does Macbeth's "life's but a walking shadow" speech mean?

Macbeth is utterly dejected by the end of the play and doesn't see much difference between winning the coming battle with the invading army or losing it. He doesn't care whether he lives or dies. What does he mean when he says that life is "a poor player that struts and frets his hour upon the stage and then is heard no more"? By "life," Macbeth--or rather Shakespeare speaking through his character--means all of us. We are all actors strutting and fretting our way through life, pretending to be something more than we know ourselves to be underneath the costumes and makeup. Some of us are better actors than others, and some not as good. But we are all acting. We are poor in the sense that we are always looking for a part, a gig. Without that role to play we are nothing. If we lose our jobs, we quickly realize our nakedness. Macbeth sees himself as an actor trying to play the role of a king, and he knows that he is just playing a part, strutting and fretting a little more than the others because he has to keep up appearances.