In act 1, scene 3, Macbeth learns the witches' prophecy that he will become Thane of Cawdor and King of Scotland. At first, this doesn't mean much to him, but after he discovers that Duncan has made him Thane of Cawdor, he gets very excited. He now thinks it is true that he will next become king.
We know he is power-hungry and wants to take control because killing Duncan to get the throne—rather than simply passively waiting for events to unfold—comes immediately to his mind. However, rather than simply having Macbeth say "I am thinking of murdering Duncan," Shakespeare
uses the literary device of imagery
—description that uses the five senses of sight, sound, taste, touch, and smell—to suggest that Macbeth is contemplating a heinous act. Macbeth states,
If good, why do I yield to that suggestion
Whose horrid image doth unfix my hair
And make my seated heart knock at my ribs,
Against the use of nature?
Macbeth describes what the "suggestion" that enters into his thoughts does to him: it "unfix[es]" his "hair." In other words, it makes his hair stand up on end. Macbeth also uses a sound image, saying that what he is thinking makes his "heart knock at" his "ribs," meaning it is pounding hard and fast. These are images we associate with fright: Macbeth's thoughts scare him because they are so terrible. It is much more powerful and creepy to show that what he is thinking is so "horrid" that it terrifies him rather than simply to say he is thinking of murdering the king.
In act 1, scene 4, when he realizes that Duncan has named Malcolm his heir, Macbeth's power hunger again comes to the forefront. Rather than back away from the desire to be king, he holds tightly to his evil thoughts of murder. Shakespeare uses the literary device of apostrophe—addressing an inanimate object or absent person—as a way for Macbeth to express his ambitions. He tells the stars to put out their lights so that they can't see his desires. He also uses the image of the eye winking or closing so it won't see the horrible thing the hand is going to do. Finally, the poetic device of rhyming couplets adds to the intensity of the scene, making Macbeth's line more memorable for us:
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.
All of the heighten the sense that Macbeth's power hunger is taking him into dark, forbidden places.