I assume you're asking about Lady Macbeth in Shakespeare's Macbeth. You don't specify, so I hope I'm correct.
Lady Macbeth is worried in this speech in Act 1.5 that she won't be ruthless enough to go through with assassinating King Duncan. She asks the spirits to unsex her (make her more like a man), fill her with the worse kind of cruelty, and thicken her blood to keep emotions such as kindness and mercy from reaching her mind and preventing her from going through with it.
After she asks the spirits to transform her, she also asks them to create fog or smoke to hide what she does from heaven, so heaven can't stop her from assassinating Duncan, either.