In this soliloquy, Macbeth has a vision in which he sees a dagger. The appearance of this dagger mirrors Macbeth's uncertainty about committing the murder. He tries to reach out for the handle and misses, just as his mind mulls over the killing of the innocent king.
Macbeth's suggestibility is also made clear through this soliloquy. When the dagger begins to point in the direction of King Duncan, Macbeth accepts that he will indeed commit the murder. The use of the word "marshal" to describe the dagger's movement suggests that it has some power over his body and his mind.
In the next few lines, however, Macbeth's state of mind changes. He becomes more rational as he dismisses the dagger as a hallucination. Instead, he blames the "bloody business" for making him see the imaginary dagger.
Finally, Macbeth urges the ground to not hear his steps as he goes to King Duncan's chamber. He does not want to get caught because he knows that if he does, his plan to become king is over. He is both ambitious and cunning here, but the final line suggests some last-minute nerves that Macbeth can barely control:
Words to the heat of deeds too cold breath gives.