When asked to analyze a quote, ask yourself the following questions: What does the quote say? What does the quote mean? Why does the quote matter?
The quote essentially says: Water will wash away the blood/murders off our hands.
The quote means: Lady Macbeth believes that something as basic as "a little water" will cleanse their conscience.
The quote matters: Here, Shakespeare reveals Lady Macbeth's psyche. From the reader's/audience's perspective it is naive to think that washing up is the same as forgetting what has been done; however, Lady Macbeth thinks that this is possible for both her and Macbeth.
Lady Macbeth's statement is not accurate. Although the evidence has been cleared, the clearing of one's conscience requires more than just a splash of water.