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.
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.