These lines are uttered by Macbeth at the very beginning of the play, specifically Act I, scene IV. At that time, the witches’ prophecy a already been revealed to him and although he already somewhat believes it, the realization of its meaning is growing within his mind, spurred by a burning ambition. However, although he has not yet decided to murder the King, he already understands that since the King already has two sons, and he names Malcolm, his elder son as The Prince of Cumberland and his heir to the throne. This of course ignites the “fire” within Macbeth’s mind since it contradicts the prophecy. Indeed, if King Duncan has already named an heir, how will he become King?
Of course, to event openly disclose such musings in front of the King, his heir and his court will immediately amount to treason for they declare that Macbeth covets the throne and that he sees the official heir as either a means for his own ambition or an obstacle to be eliminated. He therefore decides to only break the fourth wall and only tell the audience, foreshadowing the later developments of the play.
