What does it mean when the question is why does Lady Macbeth apply the spur in Macbeth?
The quotation is:
"That tears shall drown the wind. I have no spur
To prick the sides of my intent, but only
Vaulting ambition which o’erleaps itself
And falls on th’other."
In the above quotation, Lady Macbeth is considering her role in trying to persuade Macbeth to follow his ambition to become king by murdering Duncan. The line "I have no spur to prick the sides of my intent" means that Lady Macbeth knows that she herself has no real reason to want Duncan dead--she only wants him dead so that her "vaulting ambition" may push her husband into action. Later, Lady Macbeth convinces her husband that he deserves to be king, and thereby "applies the spur" to induce action. Earlier, when Lady Macbeth receives the letter from Macbeth detailing the witches' prediction, Lady Macbeth knows that her husband will likely not act on his inner desires because she thinks he is too good a person to go after what he wants. So, she knows that her ambition will have to overtake his in order to occupy the throne.