Lady Macbeth means to say that her husband is merely hallucinating when he sees Banquo's ghost, just like he hallucinated when he saw a dagger leading him to Duncan. She is reprimanding her husband for being fearful.
Lady Macbeth reprimands her husband
In lines 63-64 of Scene IV, Act III, Lady Macbeth is reprimanding (scolding) her husband for being fearful. The couple is quite ambitious and, in their pursuit to take the crown, they begin to commit crimes, including murder.
However, Macbeth soon begins to see ghosts, which his wife dismisses as mere hallucinations caused by fear. She is basically telling him to get over it, that such visions mean nothing.
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