When trying to convince Macbeth to kill Duncan, Lady Macbeth does what all loving wives do to their husbands in similar situations. She insults and berates him. Lady Macbeth questions his ability to do what he said he would do. In other words, she calls him a liar. Then she really lays into Macbeth and calls him a coward.
Wouldst thou have that
Which thou esteem'st the ornament of life,
And live a coward in thine own esteem,
Letting 'I dare not' wait upon 'I would,'
Like the poor cat i' the adage?
Of course that's not the worst of it though. Lady Macbeth then questions his manhood.
When you durst do it, then you were a man;
And, to be more than what you were, you would
Be so much more the man.
Basically, Lady Macbeth is saying "I thought you were being a man when you told me you could do it. Now you're not a man." She is being deadly serious, but it does remind me of a scene from the movie "The Sandlot." Ham Porter and the ball player from the other team are in a name calling argument. The name calling is gross and funny, but turns "deadly" serious when Porter says "you play ball like a girl." Lady Macbeth is doing a similar name calling by telling Macbeth that he must not be a man.
Macbeth still isn't totally convinced, which is why he asks what will happen if they fail. Her response is as follows:
But screw your courage to the sticking-place,
And we'll not fail.
Lady Macbeth simply tells her husband to "man up" and grow some guts. If he does that, then they cannot fail.