One of Macbeth’s arguments is what would happen if they should fail, and Lady Macbeth counters the argument by saying that if he has courage they can’t fail.
Lady Macbeth gives a brutally descriptive account of how she is tough, in order to force Macbeth to be tougher. His response is to ask: “If we should fail?” Lady Macbeth does not react positively to that!
But screw your courage to the sticking-place,
And we'll not fail. (Act 1, Scene 7)
Basically, Lady Macbeth argues that there is no way they can fail because they have a perfect plan. She wants him to kill Duncan, leave the knife near the guards after she gets them drunk so that they can’t protect him.
Lady Macbeth shows her reasons for not having faith in her husband, and he seems to also not have much faith in himself. Macbeth does not really want to kill Duncan, but he does want to be king. His wife realizes that in order for him to do the deed, she will need to push and prod him along because he doesn't have the guts or motivation to do it himself. So she both cajoles and insults him until she gets him to do it.