These are the words spoken by Macbeth after lady Macbeth persuaded and convinced Macbeth to kill the king and take the shortcut to the never-ending success of his wife and children who will inherit the throne and then become kings themselves. This quote has two different meanings. In context it means that they should have a false face that looks happy and all while their heart has a major murderous plan and that will fool the people around them in thinking that they have nothing to do with the murder. Another way of thinking about this quote is that when they hear about the death of king Duncan their faces should hide what their heart already knows, and fake being angry and sad about the death of Duncan while their heart is happy that the plan has gone the way they wanted it to go.
