In act 1, scene 5, Lady Macbeth receives a letter from her husband describing his encounter with the Three Witches and their seemingly favorable prophecies. After reading Macbeth's letter, Lady Macbeth contemplates whether or not her husband has the resolve and determination to usurp power through bloody means. In Lady Macbeth's famous soliloquy, she portrays her ambitious nature by calling upon evil spirits to make her cruel, insensitive, and murderous in order to carry out her bloody plan. Lady Macbeth beckons wicked spirits to "unsex" her so that she can behave resolute and brutal like a man. By requesting that the evil spirits "unsex" her, Lady Macbeth is commenting on the common perception of females and desires to reject her compassionate, loving nature. She proceeds to ask for the willpower to remain callous and insensitive during the bloody ordeal and once again expresses her desire to lose all her feminine qualities in favor of becoming more aggressive, violent, and hostile. Lady Macbeth then summons nature to cover her dark deeds as she completely embraces her cruel, wicked intentions.
Lady Macbeth's soliloquy is one of the most famous moments in all of Macbeth. In short, Lady Macbeth is simply asking for the strength and resolve to go through with her plan to seize the throne by conspiring in the murder of the good King Duncan. Lady Macbeth is possessed by blind ambition, lusting completely for power and prestige.
On a more figurative level, Lady Macbeth asks the spirits to "unsex" her. She is metaphorically asking the spirits to make her into a man. She tells them to "come into my woman's breasts, and take my milk for gall." Lady Macbeth is asking them to remove the aspects of herself that she considers to represent a nurturing and loving nature. Due to society's view of women, she considers violence and resolve to be inherently masculine in nature.