Explain the meaning of Macbeth's quote "is that a dagger I see before me?".

This quote is found in Act II, Scene 1 of Macbeth. Immediately before he is to murder Duncan, Macbeth sees an image of a dagger in front of him. It is covered in blood, and he takes it as a sign that he is to kill the king, telling it "thou marshals't me the way that I was going." He still wonders, however, if it is a "dagger of the mind" or something conjured by the supernatural forces (represented by the witches) that are telling him to carry out the murder. Lady Macbeth dismisses it later, but Macbeth continues to see images--the next, most significant one being the ghost of the murdered Banquo. It becomes clear to the audience that the spectral images Macbeth sees are provoked by the guilt he feels at the murders he has carried out to seize and maintain the throne. Later, Lady Macbeth, having attempted to persuade her husband throughout the play that the images should be dismissed, is overcome with guilt as well, and she spends her last scene in the play sleepwalking, attempting to wash blood stains off of her hands that are not there. So the dagger is probably a figment of Macbeth's guilt as well as a sign that guides him to the murder.