As you say, this quote is spoken by Lady Macbeth. She is speaking to her husband. Basically, she is telling him that he has to act casual and innocent even though they are going to kill the king.
The word "beguile" here means to trick or to fool. So she is telling him that he has to beguile the people who see him. He has to look like he is welcoming the king. He has to look like an innocent flower in every way so that no one will suspect that he is up to anything bad.
Lady Macbeth utters these words in Act 1, Scene 5. This is the scene in which she counsels her husband, Macbeth, on how he should behave when they welcome king Duncan and his crew. What she specifically wants Macbeth to do is to come across as a pleasant and welcoming host when the King arrives because doing so will not reveal Macbeth's true intention, which is to kill the King and take the throne.
Now, let us take a look at the quote, so that we can examine the words more closely:
Your face, my thane, is as a book where men
May read strange matters. To beguile the time,
Look like the time; bear welcome in your eye,
Your hand, your tongue: look like the innocent flower,
But be the serpent under't.
By saying that Macbeth's face is like "a book where men may read strange matters," Lady Macbeth implies that the King and his company may be able to discern Macbeth's real thoughts. Therefore, she advises him to "look like the time" if he is to "beguile the time." What this means is -- Lady Macbeth suggests that her husband must demonstrate the ability to conduct himself as others expect him to despite his "black and deep desires." He must appear "like the innocent flower," ready to protect his King and be his loyal subject.
This scene is very significant because it marks the beginning of Macbeth's unscrupulousness and tyranny. Once he kills the King, he becomes eager to kill anyone who could potentially stand in his way.