What does the following Shakespeare quote mean?
Life's but a walking shadow, a poor player,
That struts and frets his hour upon the stage,
And then is heard no more. It is a tale
Told by an idiot, full of sound and fury,
In these lines, Macbeth first claims that life is something that really lacks substance; it is only a "walking shadow." Next, he uses a metaphor to compare life to an actor, "a poor player," who has but a very short time to be on the stage (because life is so short and passes so quickly). While on stage, this actor really acts; he stalks around dramatically and emotes passionately, "strut[ting] and fret[ting]" for the audience. And then, as suddenly as the play seemed to begin, it ends, and the actor "is heard no more."
Next in this section of the speech, Macbeth compares life, via a second metaphor, to a story told by someone who lacks intelligence and common sense. Therefore, the story is rambling and ridiculous and, again, seemingly full of drama and passion, but it is ultimately meaningless and has no point, as it "Signif[ies] nothing."
The Shakespeare quotes section on eNotes has an explanation of this quotation -- which begins with Tomorrow, and tomorrow, and tomorrow...
Macbeth utters these words shortly after he is told that his wife, Lady Macbeth, has died. He is speaking of her life (the life of all humans, really) being fleeting and short. Our life is but a walking shadow (nothing we really see in substance until perhaps it is too late) a poor player (we are all bad actors...myself and my wife especially) that struts and frets his hour upon the stage and then is heard no more (we act upon the stage of life strutting and fretting and then we are gone--none of us are all that important and we are quickly and easily forgotten). It is a tale told by an idiot (the story is told by a fool...myself included...since I was led around by my wife and encouraged by the witches) full of sound and fury (while it's being told it sounds good--full of passion, full of excitement--but once the words are uttered there isn't much to it) signifying nothing (there are many words but in the end, nothing important has been said. It is all for nothing and changes nothing).
Here is a film adaptation of the scene:
Basically... it means Shakespeare was depressed when he wrote it, but also he was in the mood for writing poetry because it is one of his finest!!! Macbeth's wife's just killed herself and he's out of his depth in a bloody, murderous, political campaign mainly organised by her, without her he is lost. He knows his life has just crashed in a major way and he is alone without his partner.
Your quote is preceded by the wonderful...
Tomorrow, and tomorrow, and tomorrow,
Creeps in this petty pace from day to day
To the last syllable of recorded time,
And all our yesterdays have lighted fools
The way to dusty death. Out, out, brief candle!
In other words... "life is very short; always was, always will be"
Then we come to your selected text...
Life's a walking shadow; insubstantial, without importance or deep solid meaning; 'The Unbearable Lightness of Being'. Life's not 'real' enough.
And people are just bad, stupid actors; shouting and running about and generally making a lot of noise and fuss but not much sense, and then they die anyway.
At the end he says, The story of life is just short and absurd, full of action and events, but, in the final analysis, completely meaningless.