Macbeth is having a vision or a hallucination of a bloody dagger before he kills Duncan.
When Macbeth is deciding whether or not to kill Duncan, he is confronted with a strange sight—a bloody dagger. It is not entirely clear whether the dagger is a hallucination or if the witches are playing with him, but either way, Macbeth is stunned to see the dagger.
Come, let me clutch thee.
I have thee not, and yet I see thee still.
Art thou not, fatal vision, sensible
To feeling as to sight?
(Act 2, Scene 1)
He sees it, but he can't touch it.
Macbeth tries to decide what the dagger is first. He wonders if it is from his mind, a “false creation” of a “heat-oppressed brain.” Yet Macbeth also sees the dagger as a sign that he should kill Duncan. He considers the dagger as leading him on to the act, and it is he dagger vision that finally convinces him to act.