This quote occurs right after Macbeth learns he is to be named thane of Cawdor as part of the witches’ prophesy. He is considering whether this is a good or bad thing that the witches were right about this, because it means they could also be right about his becoming king of Scotland.
He suggests that if it this is good, he would not “yield to that suggestion” that “doth unfix [his] hair” and makes his “seated heart knock at [his] ribs” (1.3.137–39).
The suggestion he mentions is the “horrid image.” The image, then, is Macbeth’s sudden desire to get rid of King Duncan, a man whom he respects. Despite just having helped secure Duncan’s victory in battle, Macbeth feels like it could be necessary to eliminate the king in order to make way for himself on the Scottish throne. Macbeth is disturbed because this thought goes against his moral code, and yet he cannot help himself from thinking it. This foreshadows that Lady Macbeth will easily persuade her husband to kill Duncan, since he has already thought about it before.