What is bothering Macbeth. What does he think he sees?

Macbeth is "bothered" by his plan to kill Duncan, as he sleeps. Macbeth imagines that he sees a bloody dagger pointing toward Duncan's chamber. Frightened by the apparition of a "dagger of the mind," he prays that the earth will "hear not [his] steps" as he completes his bloody plan (38, 57).