What is the significance of this quote from Macbeth?

Well, earlier in the play, Macbeth is greatly anguished by the blood on his hands from his killing of the King. He feels tremendous guilt over having commited the murder. Lady Macbeth tells him that "little water clears us of this deed"; she means literally to wash away the signs of blood. However, later in the play when she obsessively washes her hands she realises the guilt that the blood symbolises needs more than water to be cleansed away.