After Macbeth has committed the murder of King Duncan, he returns to his bedroom to meet his wife. When she discovers that he has left the room with the murder weapons, the bloody daggers, still on his person, she knows that she must return them to the room where his chamberlains sleep in order to frame them convincingly. When she leaves Macbeth alone, he says,
Will all great Neptune's ocean wash this blood
Clean from my hand? (2.2.78-79)
Macbeth alludes to Neptune, the Roman god of the seas, and asks if the entire ocean would be enough to wash the blood off his hand. He is using hyperbole, or exaggeration, in order to emphasize how much guilt he feels for the murder and, also to describe the amount of blood that is literally on his hands. This leads to the theme that guilt is harder to get rid of than we might believe. We see this theme with Lady Macbeth as well.
Then, after Macduff discovers Duncan's dead body in the morning, he tells the others to
Approach the chamber and destroy your sight
With a new Gorgon. (2.3.82-83)
He alludes to Medusa, the Gorgon: a mythological creature who turned people to stone just by looking at them. Macduff suggests, by the use of this allusion, that the sight of Duncan's dead body is so awful that it could petrify you, just like Medusa's gaze.