As always, to identify the meaning of any given individual quote it is vital to look at that quote in context and try to use those contextual clues to help us understand it. This quote is actually from Act I scene 3 and is said by Macbeth as part of an aside in response to hearing the prophecy of the witches and then having the first element of that prophecy completed through his gaining of the title of Thane of Cawdor. In this aside, Macbeth explores his curious range of emotions at having had the first stage of the prophecy confirmed, and debates whether the prophecies he has received are good or bad. Note what he asks:
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?
Macbeth is asking if the prophecies are good, why is it that he finds himself contemplating killing King Duncan, something that terrifies him incredibly because it is such an unnatural thing to want to do? This quote therefore points towards the way in which Macbeth is already contemplating committing regicide to gain the crown for himself.
This quote is taken from Act I, Scene 3, of Macbeth when Macbeth and Banquo meet unexpectedly with the witches and receive some important prophecies. As predicted, Macbeth becomes the Thane of Cawdor which makes him wonder if the final prediction, that he will become King of Scotland, will also come true. His mind immediately turns to murder but he is horrified at the prospect of committing such a crime. In fact, he refers to this proposed murder as a "horrid image" and he emphasizes his disgust with an image of his hairs standing on end ("doth unfix my hair") and his heart beating hard and fast inside his chest ("make my seated heart knock at my ribs"). The thought of murder, therefore, is causing him considerable anxiety.
This quote, then, represents an important conflict in Macbeth's life. His inner sense of ambition tells him to murder King Duncan and take the throne but his body is not reconciled with such an idea. Over the next few Acts, however, his ambition will triumph and set Macbeth on a murderous and destructive path.