Deliberate exaggeration, which no one takes literally, to give emphasis to a feeling or a situation.
In Macbeth
Will all great Neptune's ocean wash this blood / Clean from my hand? No, this my hand will rather / The multitudinous seas incarnadine, / Making the green one red.
Macbeth exaggerates when he says that the whole "ocean" would not wash away the blood and that his hands "will dye the many seas" red. No one takes this literally, but the exaggeration measures the weight of the crime.