The line, firstly, employs two examples of alliteration. Alliteration refers to the repetition of the same sound or syllable in consecutive words, as in "let light" and "deep desires." In this instance, the technique is used to accentuate the profundity of Macbeth's passion in committing a most heinous act—regicide. Macbeth's "vaulting ambition" is to kill King Duncan and usurp the Scottish throne. In the process, he will override all the rules required for natural ascension.
In addition, the phrase "let light not see" is a metaphor in which light is compared to an ever-present and constant witness that can see all. Furthermore, light is a symbol of goodness and understanding. Macbeth's wish is that light should be blind so that it cannot perceive the evil he is contemplating. In this way, what he plans will remain unknown, and his complicity in the terrible deed he plans to commit will remain undiscovered.
The use of the word "black" is, similarly, a metaphor for the profound evil that has overwhelmed Macbeth. The fact that he calls his desires "black" clearly symbolizes that he means no good. Because these desires are "deep" emphasizes the fact that Macbeth's lust for power has vested itself into his soul and consciousness. He is completely committed to achieving his malicious objective.
The clever juxtaposition of light and dark in the line also exposes the turbulence that Macbeth has been experiencing. The contrast highlights the fact that Macbeth knows that his thoughts are evil and therefore wrong, but he has become so overwhelmed by his ambition that he has allowed malice to overrule reason and virtue.
In the end, Macbeth follows through with his plan and murders his king. He claims the throne and his rule introduces a period of utter depravity in which his paranoia and ruthless thirst for blood becomes the order of the day. Ultimately, it is Macbeth's malice that leads to his self-destruction and almost ruins Scotland.
Macbeth's ambition has been awakened by the Weird Sisters' prediction that he will become king, so when King Duncan names his older son, Malcolm, the Prince of Cumberland and his heir to the throne, Macbeth's thoughts turn dark. He says,
Stars, hide your fires;
Let not light see my black and deep desires. (1.4.57-58)
In these lines, Macbeth employs a figure of speech called apostrophe. Apostrophe is when the speaker addresses something that is not alive and cannot respond as though it is alive and could talk back. Macbeth speaks directly to the stars, asking them to go dark so that no one and nothing will be able to see signs of his dark ambition. The use of apostrophe does personify the thing the speaker addresses in a very minor way, but only insofar that it attributes to that thing the ability to respond.
Ultimately, these lines also help to show the audience the beginning of Macbeth's transformation from loyal and brave to traitorous and tyrannous. He knows that he is thinking evil thoughts, and rather than try to repress them, he asks the stars to go dark so that he will be better able to hide them.
This phrase actually contains personification of stars and a metaphor for evil.
“Stars hide your fires” is personification. The stars are being asked to give Macbeth darkness, so no one can see his “black and deep desires.” Calling his desires black and deep is a metaphor, because the thoughts are not literally dark, but he is saying they are dark because they are evil. Here “dark” is a metaphor for evil. His thoughts are evil because he needs to commit murder in order to get what he wants.
In Act I, Scene 5, Macbeth discovers that he has been named Thane of Cawdor, as the witches promised, but he has not been named Duncan’s successor. Macbeth says that the prince is in his way.
The Prince of Cumberland! That is a step(55)
On which I must fall down, or else o'erleap,
For in my way it lies. (enotes etext p. 18)
The current prince, Malcolm, is in his way. Macbeth needs to be king. He needs to kill Duncan and get rid of Malcolm, the prince, in order to do it. This is the evil and dark thought he does not want anyone to see.