This comment comes shortly after Macbeth has told Lady Macbeth that King Duncan is arriving for a visit to Inverness, their castle home, later that evening. Macbeth is hungry for power, ultimately hoping to be named and crowned as the King of Scotland, so this seems like an opportunity to remove an obstacle to the realization of his goal.
However, Lady Macbeth doesn't trust her husband to be able to conduct himself without giving away his true desires and thoughts about King Duncan. She encourages him to be a respectful and gracious host to King Duncan, taking care that his facial expressions and actions not give any indication of their plans against the king.
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
She wants him to appear as innocent of any deceit or evil thought as a flower, while remembering that he is actually the poisonous serpent ready to strike behind the pose of innocence.
Just before the arrival of king duncan lady macbeth suggests macbeth to look chearfull and happy.she asks him to act talk and look as if he is very happy to be the host of king duncan, peolpe should not even get the slightest of doubt about their planning. she asks macbeth to hide his real nature beneath a look as innocent as a flower which contains poisonous vectar. thus , over all she asks him to disguise himself and hide his real feelings .