In a play in which ambition and power were the only goals of the main characters, Macbeth realizes that all of his decisions and actions are meaningless: life is a "brief candle" set to go out. Not only does Macbeth realize that life is meaningless, he begins to see his life as ruled by others. This is a take on the Shakespearian trope of "all the world's a stage;" but rather than highlighting the performed nature of identity or love, Macbeth uses this theater metaphor to show that our ambitions and actions are part of a badly scripted performance without meaning. In other words, life, ambition, achievement are all illusions that dissolve in death. This is one of the most famous speeches from this play and it has inspired multiple literary and artistic works, including William Faulkner's 1929 The Sound and The Fury.

Macbeth, enraged at learning how the "fiend" deceived him, abandons his plan on letting the enemy waste their strength on trying to take his castle. Instead of this prudent plan, he decides to meet them on the field of battle in a desperate (and suicidal) effort. Notice what happens to many of his men when they are given the chance to desert once in the open field.

Here, Macbeth realizes that the witches and the fiend, or devil, who inspired them to raise the apparitions for Macbeth, have deceived him. "Equivocation" refers to language that is ambiguous in order to conceal the truth, and while Macbeth considered the Woods of Birnam moving against him an impossibility, he sees now that it was merely symbolic and that Malcolm's army "moves" the forest with their covering branches.

Macbeth is considering the meaning of life and thinks back on all of his "yesterdays" and what they have led to. In a moment where he is possibly contemplating suicide, Macbeth compares his life to that of a candle—a brief amount of time that is now spent.

Continuing with his nihilistic view on life, Macbeth uses this expression to say how all past events have "lighted" (that is, guided) fools to their death.

The adjective "petty" means that something is trivial or unimportant. Macbeth considers the endless march of days to be meaningless, without purpose or direction.

Many critics have contended the meaning of this line, whether Macbeth is callously indifferent to his wife's death or whether he is so sunk in misery by the news that he finds life not worth living. Perhaps the most plausible reading of this is connected to the latter and that Macbeth wishes that she had died after the battle, when there would have been time to mourn her properly.

"Ague" in this context can either refer to a kind of high fever or a disease characterized by fever, intense cold, and shivering, or it refers to a state of fear, distress, or other strong emotion. The association with famine perhaps indicates that former definition is the more likely, but in either case, both would make soldiers abandon a castle siege.

"Tomorrow and tomorrow and tomorrow" has eleven syllables. The effect of the extra syllable at the end of the iambic pentameter line may be to suggest that the "tomorrows" continue to be repeated in Macbeth's mind, as:

Tomorrow and tomorrow and tomorrow and tomorrow and tomorrow and tomorrow and tomorrow and tomorrow...

Note that Shakespeare plays on the extra syllable in the first line when he refers to the "last syllable" of recorded time. It is as if all the "tomorrows" are trying to catch up with that last syllable "ow" at the end of the first line.

Harold Bloom argues that Macbeth's response to his wife's death is nihilistic. In Macbeth's world, there is no life after death (541).

Bloom, Harold. Shakespeare: Invention of the Human. New York:  Riverhead Books, 1998. Print.

William Faulkner borrowed the phrase "sound and fury" for the title of one of his best novels, The Sound and the Fury. Faulkner had a pessimistic view of life and human nature himself. "When [Malcolm] Cowley, for example, wrote asking if it would be fair to call his work a 'myth or legend of the South,' Faulkner testily replied that the South 'is not very important to me,' adding, in a gratuitous discharge of bile, that in his opinion human life is 'the same frantic steeplechase toward nothing everywhere and man stinks the same stink no matter where in time.'" Frederick Crews, "Faulkner Methodized."

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 words "struts," "frets," "hour," "upon," etc., are stressed in the meter of 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.

With the last two words, “no more,” it is as if the sounds of the shadow’s pacing fade out. This is because of the strong “O” sounds in “no” and “more” and also because the word “no” is not naturally stressed. The metrical stress, if anywhere, would be on the word “more”—almost like an echo of the departed shadow’s footsteps. A huge shadow has paced across the stage and disappeared on the other side. 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.)

Why does Macbeth use the word "fools" to describe all those who have died before him? They were fools because all their plans and activities were canceled by death.

The word "should" can be misleading for many modern readers. Macbeth does not mean that he would have preferred to have his wife die later, that she "ought to" have died hereafter, or that it would have been more convenient if she had died later. He is using the subjunctive mood, which is gradually disappearing from the English language. According to the article in Wikipedia on "Subjunctive":

The subjunctive is a grammatical mood found in many languages. Subjunctive forms of verbs are typically used to express various states of unreality such as wish, emotion, possibility, judgment, opinion, necessity, or action that has not yet occurred; the precise situations in which they are used vary from language to language. The subjunctive is an irrealis mood (one that does not refer directly to what is necessarily real) – it is often contrasted with the indicative, which is a realis mood.

A modern American would say:

"She would have died hereafter" or perhaps "She would have died sooner or later."

Macbeth is not showing the slightest concern about his wife. No doubt he once loved her, but it seems as if he has no love for her at all anymore. In fact, he probably hates her because she talked him into killing King Duncan when he really didn't want to do it. If he hadn't listened to her he could now be enjoying a comfortable and honorable old age. He holds her responsible for the mess he is in. Their marriage has evidently fallen apart. The fact that they share the same guilt does not bring them together but drives them apart. Each reminds the other of their shameful deed. Everybody in Macbeth's kingdom hates him. He will probably lose the coming battle with the English army, in which case his own death is inevitable.

The messenger is frightened and confused, afraid to tell Macbeth what he saw and still bewildered by the sight of what appeared to be a moving forest. He tells Macbeth, "I look'd toward Birnam, and anon, methought..." The word "methought" indicates that he still doesn't know whether he saw the trees moving or whether he was having an hallucination. Macbeth can't believe it either and immediately calls the Messenger a liar.

Macbeth is planning to remain inside his castle and withstand a siege. This is probably a good idea, under the circumstances, because he is continually losing men by desertion. He would lose more men if he ventured outside his castle with his remaining forces, because they would be free to go over to the enemy.