This is an extremely important line and concept in Macbeth. This line comes when Macbeth has just killed Duncan and Lady Macbeth is helping him wash his hands of the blood. Directly before she says this, Macbeth says:
How is't with me, when every noise appals me?
What hands are here? ha! they pluck out mine eyes.
Will all great Neptune's ocean wash this blood
Clean from my hand? No, this my hand will rather
The multitudinous seas in incarnadine,
Making the green one red.
He doesn't believe that his hands will ever come clean. Lady Macbeth, in return says:
At this point, she is the calm and collected one. Macbeth is losing it with the guilt. Later on, the tides turn, as you'll see.
Water washes off Duncan's blood from Macbeth easliy. Macbeth, after bouts of inner struggle, has murdered Duncan but is now overcome with guilt and fear. Lady Macbeth must see to completing the conspiracy by planting the dagger in the possession of unsuspecting servants. Here the masculine and feminine roles reverse in that Lady Macbeth is the aggressor and Macbeth is squeamish about the act he just committed. Upon her return she casually notices the blood on her hands, as oppossed to Macbeth's obessession with blood on his hands, literally. Lady Macbeth's statement "A little water..." also contradicts Macbeth's earlier quote, "Will all great Neptune's oceans wash the blood from my hand? No..." The quote in question clearly reveals the Macbeths' approach to Duncan's murder. Macbeth in his guilt is convinced he will be found out, and Lady Macbeth in her ruthlessness is only concerned is hiding the act. Both equally guilty but obvioulsy two opposing attitudes toward the crime. Macbeth cares that he is guilty. Lady Macbeth, though only an accomplice, is unfeeling that any wrong doing has been done.