What does "False face must hide what the false heart doth know" mean?

"False face must hide what the false heart doth know" is the very last line of the very first act in Macbeth. 

It is uttered by Macbeth, who has finally been convinced by Lady Macbeth to engage in a plot to murder to Scottish King, Duncan, in order for Macbeth to eventually become the ruler of Scotland. In the scene, Duncan and everyone else are enjoying a welcome feast at Macbeth's castle, where they are visiting. Macbeth and Lady Macbeth steal away for a moment to talk. When Macbeth had first heard the three witches / weird sisters' prophecy that he would become Thane of Cawdor and then King of Scotland, he immediately felt greedy for those positions. His ambitious feeling began to well up over his good sense. However, his moral code fought against it and he tried to convince himself that evil deeds would not be worth it. He attempted to battle his evil ambitions. When he goes home to his wife, though, her resolve is absolutely steelier than his. She is 100 percent sure that murdering Duncan is the quickest and best path to making her husband King and herself the queen. She manipulates, insults, and coerces him into eventually agreeing with her. She uses their sexual passion and his fear of being "unmanly" and cowardly to make him agree to kill Duncan. 

When King Duncan arrives at Macbeth's castle for a visit, he is jolly and optimistic as usual. He doesn't know that his seemingly gracious hosts are plotting his death. In the last scene (scene 7) of the first act, Macbeth shows that he is finally convinced to commit an evil deed by saying this line. He agrees with Lady Macbeth that they must outwardly hide (false face must hide) their secret evil plans (which are from their false heart). Although they know what they plan to do, the success of the plan relies on them acting like they are in grief and anger when Duncan is discovered to be murdered.