Evidently you are referring to Macbeth's speech in Act 2, Scene 1, beginning with: "Is this a dagger which I see before me, / The handle toward my hand?" Macbeth undoubtedly sees a dagger, but it must be an hallucination. He says, "Come, let me clutch thee." But he is unable to do so. This would seem to prove, if any proof were necessary, that it is, as he says, "a dagger of the mind, a false creation / Proceeding from the heat-oppressed brain." Macbeth seems subject to mental aberrations, including trance-like states and hallucinations, as when he imagines he sees Banquo sitting in his place at the banquet table. If the dagger were real, Macbeth would be able to use it to murder King Duncan, but according to the stage directions "He draws his dagger." Later his wife pooh-poohs his "fatal vision," saying, "This is the very painting of your fear. / This is the air-drawn dagger which you said / Led you to Duncan." (3.4.74-6)
In Act II, Scene 1, Macbeth sees a dagger, which appears to be directing him toward the bedchamber of King Duncan. This dagger is very likely to be a hallucination since Macbeth already has a dagger in his pocket, which he takes out and observes in the middle of this soliloquy. Moreover, the dagger is the physical manifestation of Macbeth's increasingly fragile state of mind. Remember that Macbeth was initially reluctant about committing the murder of King Duncan and that it took some persuasion from Lady Macbeth. Moreover, in Act I, Scene 3, Macbeth was made very anxious by the mere thought of killing Duncan: he said that such a murder "shakes so my single state of man."
Now, in Act II, Scene 1, Macbeth is suffering from what he describes as a "fevered brain," caused by a combination of fear and anxiety. While the appearance of the dagger briefly distracts him from the murder, Macbeth soon realizes that he must ignore this hallucination and move quickly before he loses his courage:
Whiles I threat, he lives.
Words to the heat of deeds too cold breath gives.