An aside is a remark by a character on stage that is only meant for the audience to hear. Even though there are other characters on stage, the audience is expected to accept the idea that those characters do not hear the aside.
In Act I of the Shakespeare's Macbeth, Macbeth is genuinely shocked to hear a prophecy that he will be crowned King of Scotland. However, it becomes evident that he likes the idea of power, and before long he begins scheming.
Near the end of Act I, Scene 4, Macbeth hears the King name his son Malcolm the Prince of Cumberland. Knowing that Malcolm is next in line to the throne, Macbeth utters the following aside:
The Prince of Cumberland! That
is a step
On which I must fall down, or else o’erleap,
For in my way it lies. Stars hide your fires;
Let not light see my black and deep desires:
The eye wink at the hand; yet let that be
Which the eye fears, when it is done, to see.
This statement shows that Macbeth realizes that Malcolm is in his way to the kingship and must be dealt with. Macbeth knows that he is being treacherous, as he reveals with the words, “Let not the light see my black and deep desires.”
The aside shows an important change in Macbeth’s character. From here forward, his focus will be on becoming king, no matter what he must do. He will not let morals or principles get in the way of his ambition.