Macbeth was told by the Weird Sisters that he would become Thane of Cawdor and King of Scotland, and then, lo and behold, Ross and Angus arrive, bringing the news that he has, indeed, been named Thane of Cawdor. Now Macbeth really begins to believe, in earnest, that the more significant prophecy could come true, and he begins to entertain ways in which it might. He says,
I am thane of Cawdor.
If good, why do I yield to that suggestion
Whose horrid image doth unfix my hair
And make my seated heart knock at my ribs,
Against the use of nature?
In other words, he is now Cawdor, as the Weird Sisters said he would be. However, if this is a good thing, he asks himself, then why does he now find himself considering the murder of the current king, which is such a horrible idea that it makes his hair stand up on end and his heart pound loudly within his ribcage in such an unnatural way? Macbeth is already considering the nearest way to the throne, and he is unnerved by how quickly he has jumped to regicide as a possibility.