What does Lady Macbeth mean by the line: ''... look like the innocent flower, but be the serpent under it"?
This line is part of a speech that Lady M makes in Act One, Scene five. She is trying to convince Macbeth to become a villain and murder King Duncan. She encourages him to play the fabulous, welcoming host to the King, so that no one will suspect his true intention -- murder.
She notes that his face gives away his inside feelings:
Your face, my thane, is as a book where men
May read strange matters.
This is significant in the course of the action of the play, because once Macbeth turns to a villainous course of action and begins to hide his "serpent" behaviour behind a veil of niceness and false innocence, he gets deeper and deeper into the crimes he must commit, including the murder of his friend Banquo.
It is also significant in that Lady Macbeth is the real brains behind the murder of Duncan, but Macbeth is the one that actually carries all the crimes out. So, in his heart, was Macbeth really a villainous murderer, willing to commit any act for the power of being King? Or, was he led astray by his wife?
You could even consider a parallel between Macbeth being swayed by Lady M and Adam being swayed by Eve in the Garden of Eden. The mentioning of the serpent in this text is a nice reminder of who the real villain was in Eden. If Macbeth had stopped to consider this parable, he might have realised that he was heading to his own demise.