When Macbeth sees the ghost of Banquo at the banquet, he starts yelling at him:
Thou canst not say I did it. Never shake
Thy gory locks at me (4.3.54–55).
As no one else can see the ghost, his guests immediately think there's something wrong with Macbeth. They could be forgiven for thinking that the king is taking leave of his senses.
Somewhat diplomatically, Ross says that Macbeth isn't well. Lady Macbeth, who's convinced that her husband shouldn't display signs of weakness in front of his nobles, quickly tries to get a grip on a potentially embarrassing situation.
She bids the assembled guests sit down and assures them that her husband often acts like this and has done so since he was a child. The best thing for them to do, she cautions, is to ignore him. Otherwise, they're liable to make him angry. Given that Macbeth, by this stage in the play, has developed into a bloodthirsty tyrant, that's probably wise under the circumstances.
But whatever she might say to her guests, Lady Macbeth isn't pleased by her husband's odd behavior. She thinks it makes him look weak, not to mention unmanly. Speaking to Macbeth so that no one else can hear her, she flat out asks him if he's a man.
Once again, Lady Macbeth is trying to get her husband to do what she wants by calling into question his manhood. This is the exact same approach she adopted when Macbeth appeared to be getting cold feet over killing Duncan.