Essentially, this line is Lady Macbeth's warning to her husband in how to engage in immorality to get what they both want. She says this to him in Act I, sc. 5, when he tells her that Duncan is coming. Lady Macbeth recognizes this as the perfect opportunity for Macbeth to kill Duncan and seize power. In order to become King, Lady Macbeth realizes that Macbeth must kill Duncan. She also understands that he lacks the vocabulary and full understanding to do so. As a warrior and a fighter on the battlefield, murder is done in name of King and country and there is little duplicity involved. The slaughter is understood. Lady Macbeth shrewdly realizes that her husband might need some level of guidance in how to murder for personal gain. It is in this where she advises him to "beguile time," and put on pretenses as if he is a gracious host, and devoid of any malicious intent. The "serpent under it" is how she believes Macbeth will best understand what needs to be done in how the murder should be executed. In this line, Lady Macbeth's initial deviousness is evident, something that will change over the course of the play. At the same time, Macbeth's overall innocence is also evident, something that will also change over the course of the play.