This brief speech, spoken as an aside while Banquo, Ross, and others are still on the stage, demonstrates the extent to which the prophecies of the witches have kindled Macbeth's ambition, even as he remains deeply ambivalent about the possibilities that may come of his ambition. He has just discovered that, true to the witches' prediction, he has been given the title of Thane of Cawdor. He is still ambivalent about the prophecy, claiming that it "cannot be ill, cannot be good." On the one hand, he has seen that he now is "earnest of success" in succeeding to the throne. On the other, he recognizes that to actually carry out the final stage of the prophecy, he will have to carry out deeds which:
Whose horrid image doth unfix my hair
And make my seated heart knock at my ribs,
Against the use of nature?
In short, this speech reveals a man who has just discovered that he is destined for great things in life. At the same time, he realizes that the achievement of these things carries some frightening implications, and he is very torn. While he later follows this speech with an aside that "if chance will have me king/why, chance will crown me/Without my stir" it is clear that he is deeply conflicted. His wife will show no such ambivalence when he tells her of the news.
In this soliloquy, Macbeth is reacting to the witches' prophecies. At first, he is confused: the witches told him of two prophecies which have come true, like becoming the Thane of Cawdor, but he does not know how his rise to the throne ("imperial theme") might happen.
It does not take Macbeth's mind long to start thinking about murdering King Duncan. This implies that Macbeth is rather suggestible. Note, for instance, how the word "yield" is used here to show that Macbeth is easily swayed. In addition, it becomes clear that, deep down, he has always longed for more power. His ambition, therefore, has always existed; it just needed someone (or some particular prophecy) to bring it to the surface.
Finally, that Macbeth's mind switches so quickly to committing murder also indicates that he is not afraid of violence. In fact, he is prepared to use physical violence to exert himself, if he believes that the prize is worth the fight.