In Macbeth, what does Macbeth mean when he says, "Stars hide your fires"?
Macbeth speaks this quote as part of an aside in act 1, scene 4. So far in the play, Macbeth and his friend Banquo have won a battle against Norway and the traitorous Thane of Cawdor. On their way back from the battle, they meet three witches on the heath; the witches hail Macbeth as Thane of Cawdor and future king. Macbeth is confused because he thinks the Thane of Cawdor (the traitor) lives, but he is told later in the scene by Duncan's men that he has been executed. Macbeth has already begun to think about how he will become king, and quickly. The aside in scene 4 provides further evidence that Macbeth will try to gain the crown by violent means.
In scene 4, Macbeth has been called to Duncan to receive his new title, Thane of Cawdor. In the same scene, Duncan formally names his eldest son, Malcolm, as his heir. Since Macbeth is already considering murdering Duncan to take his place, Malcolm presents another obstacle in his way to the throne. He calls Malcolm "a step / On which I must fall down, or else o'erleap." (lines 50-51) This means that Malcolm will either keep him from the crown or he will have to find a way to overcome the heir. Immediately after these lines, Macbeth's aside continues, "Stars, hide your fires; / Let not light see my black and deep desires." (lines 52-53) There are two relevant meanings here. One, Macbeth is wishing for darkness to hide his murderous plans and eventual actions. So he literally is asking the stars not to shine so that his thoughts will not be visible (this is also why he's speaking in aside here). Second, the "stars" and "fires" could also represent Macbeth's ambitions, which he wants to keep hidden from the other characters.
This aside is an early piece of evidence that Macbeth has come up with the plan to kill Duncan on his own even before conferring with his wife, who is often seen as the instigator who pressures her husband into murder for her own selfish ambition.
In Act 1 Scene 4 of Macbeth, Macbeth says, "Stars, hide your fires!" in an aside. Duncan has just named his eldest son Malcolm the Prince of Cumberland, so Macbeth realizes that the path to him becoming king is now blocked by Malcolm who is next in line--even if Macbeth is successful in killing Duncan, he now needs to find a way to get rid of Malcolm too. Macbeth understands that he must either "fall down" and let Malcolm become king or "o'erleap" and take fate into his own hands by killing Malcolm too. Macbeth is already full of greed and ambition, and he truly desires to be king, so he says, "Stars, hide your fires!" so that his "black and deep desires" are not suspected by anyone else. Macbeth wants to continue masking his true desires so that no one suspects him guilty of the murderous acts he will soon commit.
Macbeth is thinking about how he wants to be king. He knows that he will have to kill Duncan in order to do that. "Stars, hide your fires" refers to Macbeth wanting to hide his desires and intentions from both others and himself. He wants the blanket of darkness to hide the fire of malice inside of him. Macbeth knows that murder is wrong but he wants to feel that he is good a person. The night that’s illuminated here is not only the physical darkness of the sky but also the internal darkness of not facing up to one’s own immoral longings.