This quote from Shakespeare's Macbeth comes after Lady Macbeth has received a letter from her husband and before he arrives at the castle himself.
The letter details the predictions made by the weird sisters, and informs Lady Macbeth that one of the predictions has come true: Macbeth has been made Thane of Cawdor. The second prediction is that Macbeth will be king.
Lady Macbeth is extremely ambitious, as is her husband, and in her speech after she reads the letter she questions whether or not her husband is ruthless enough to do what's necessary to obtain the throne--kill Duncan.
The section of the speech you quote above is in a speech that's given just a few minutes later and shows Lady Macbeth preparing herself to be ruthless. She wants to be the stereotypical male, aggressive and ruthless, masculine. The famous line, "unsex me here" is Lady Macbeth asking to be rid of any female qualities, such as gentleness and pity. She wants to be filled with cruelty, and wants her blood to be made thick. The idea of the thick blood is that thick blood will stop those same female qualities from reaching her mind.
In summary, then, Lady Macbeth asks the "spirits" to strip away female qualities and replace them with male qualities, those qualities that will enable her to be ruthless enough to assassinate King Duncan.