If motion pictures had existed in Shakespeare's time he might have said that life is like a black-and-white movie, because the actors we see on the screen really are "walking shadows." Shakespeare seems to be saying that we are all like actors on the stage but each of us is carrying a candle which casts a shadow on the floor. When our candle burns out, we cease to exist. We are as insubstantial as shadows.

Notice how the italicized words are emphasized in the following lines to create the impression of a gigantic shadow pacing across a stage:

...a poor player 
That struts and frets his hour upon the stage, 
And then is heard ... no more.

But the last two words are not emphasized! . . . . It is as if the sounds of the shadow's pacing fade out, quite appropriately, with "no more." This is because of the strong "O" sounds in "no" and "more" and also because the word "no" which comes next in sequence to the pacing words is not naturally emphasized, but the emphasis, if anywhere, would be on the word "more"--almost like an echo of the shadow's footsteps. A huge shadow has paced across the stage. These wonderful words can be much better appreciated if one reads them aloud. (But, for that matter, all of Shakespeare's words can be better appreciated if one reads them aloud, because they were intended to be spoken aloud.)

Shakespeare may have directed the actor playing Macbeth to be walking back and forth on the stage while speaking all the lines beginning with "Out, out, brief candle." In that case the actor's heavy steps on the boards would correspond with the emphasized words in the soliloquy, and the actor himself might be regarded by the audience as a walking shadow. If Macbeth is actually pacing back and forth like a walking shadow as he says these lines, he is brought to a halt by a messenger who has come to tell him that he just saw Birnam Woods starting to move!

Macbeth likens “life” to a “walking shadow.”

At this point in the play, Macbeth is quite dejected.  He has just learned that his wife has killed herself.  His response to her death is rumination on just what life is. 

Everything seems to be against Macbeth at this point.  His men are deserting him.  Malcolm’s army is coming for him.  There is a general sense of unrest and disappointment about him.  He realizes that he does not have many options left.  In a way, this speech is his acknowledgement that he is not going to live much longer either.

She should have died hereafter;

There would have been a time for such a word.(20)

Tomorrow, and tomorrow, and tomorrow

Creeps in this petty pace from day to day

To the last syllable of recorded time … (Act 5, Scene 5)

A shadow has no substance.  It is a poor substitute for the real thing.  At this point, Macbeth feels this way about life.  He could have had so much and done so much.  Instead, he was left at the end with a wispy nothing.

It is clear that Macbeth is worried about his legacy.  He frets over the concept of “tomorrow” and describes life as a “poor player” who has only his hour upon the stage.  Everything he hoped to accomplish, and his hopes for his line, have come to nothing. 

He needs to accept this, but he also uses it as a justification of his own impending suicide of sorts.  He will fight to the death, regardless of how foolish his actions are.  He refuses to admit to himself that it’s over, and he wants to go out in a blaze of glory.