king. Once her servant brings word of the king's imminent arrival and leaves, Lady Macbeth reveals her malevolent desire to become queen in her passionatesoliloquy:

Come, you spirits That tend on mortal thoughts, unsex me here, And fill me from the crown to the toe top-full Of direst cruelty. Make thick my blood. Stop up the access and passage to remorse, That no compunctious visitings of nature Shake my fell purpose, nor keep peace between The effect and it! (Shakespeare, 1.5.30-37)

Essentially, Lady Macbeth is instructing evil spirits from hell to consume her soul and transform her into a callous being, who lacks the conscience of a man or woman. She desires to become a malevolent, hostile individual without remorse in order to completely focus on her evil purpose. Lady Macbeth also requests for "murd'ring ministers" to transform her mother’s milk into poisonous acid, which emphasizes her motivation to shed her gentle female qualities in favor of cruel, hostile traits. She then instructs the thick night to conceal her evil deeds by covering the world in darkness so that her "keen knife see not the wound it makes."

Overall, Lady Macbeth's passionate soliloquy emphasizes her ambitious nature and reveals the extent she is willing to go to become queen. Lady Macbeth's speech also characterizes her as a malicious, calculating woman, who is determined to convince Macbeth to assassinate King Duncan.

It’s act 1, scene 5, and Duncan is on his way to Macbeth’s castle—right into the deadly trap set for him by Lady Macbeth. News of Duncan’s imminent arrival has made Lady Macbeth giddy with excitement. She senses that she and her husband are about to fulfill their destiny.

But before she can put her wicked murder plot into effect, Lady Macbeth needs a little help from the forces of the supernatural. She invokes those spirits that assist murderous thoughts to “unsex” her—that is, make her less of a woman and more like a man, and fill her up from head to toe with cruelty. She also wants the “murd’ring ministers,” as she calls them, to clog up her veins and thicken her blood so that she will feel no remorse for what she’s about to do.

This apostrophe by Lady Macbeth is by far the best scene in Act I. After she reads Macbeth's letter that prophesies of him becoming king, she walks upon her ramparts and speaks to the "spirits that tend on mortal thoughts" to completely remove her human qualities; this notion is revealed in the request to be "unsex[ed];" she wants to be neither a man nor woman, for both sexes have consciences, and she doesn't want one, for it will potentially inhibit her from her desire, which is to kill Duncan so that Macbeth can become king. The fact that she wants to be some kind of evil creature who does not have a conscience can be seen in her request that the spirits "make thick [her] blood. If her blood is thick, then she is dead, and if she is dead, she becomes some kind of spirit. The spirit that she desires to become is one that is filled with a "direst cruelty." This soliloquy reveals her nefarious ambition.  

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.

You have told us yourself where in the play this happens.  At this point in the play, Macbeth has already heard the witches' prophecies about him.  He has already been made Thane of Cawdor.  But he is not yet king.  The king, Duncan, is coming to visit Macbeth's castle.

In this quote, Lady Macbeth is wishing to be "unsexed" so that she could be more cruel and power hungry.  This refers to the idea that women should not have these traits.  But she thinks that her husband will not be ruthless enough to do what he needs to become king.  So she wants to be ruthless enough to push him to kill Duncan.

Let's look at the context of this line. In act 1, scene 5, Lady Macbeth reads a letter from her husband, telling her of the events we witnessed in the previous scenes: Macbeth met three witches in the woods, and they hailed him as Thane of Glamis (which he is), Thane of Cawdor (which he becomes), and king hereafter (prophesying a future crown for him). As soon as she finishes reading, a messenger tells her that King Duncan will be coming to her castle tonight. The messenger leaves, and she speaks the following famous lines:

The raven himself is hoarse
That croaks the fatal entrance of Duncan
Under my battlements. Come, you spirits
That tend on mortal thoughts, unsex me here,
And fill me from the crown to the toe top-full
Of direst cruelty! make thick my blood;
Stop up the access and passage to remorse,
That no compunctious visitings of nature
Shake my fell purpose, nor keep peace between
The effect and it! Come to my woman's breasts,
And take my milk for gall, you murdering ministers,
Wherever in your sightless substances
You wait on nature's mischief! Come, thick night,
And pall thee in the dunnest smoke of hell,
That my keen knife see not the wound it makes,
Nor heaven peep through the blanket of the dark,
To cry 'Hold, hold!'

The raven croaking the "fatal entrance of Duncan" shows us what Lady Macbeth is scheming. She is ambitious, and wants her husband to become king as the witches foretold. She sees Duncan's visit as a fateful opportunity. In order to follow through with the assassination though, she will need to be tough. She asks the spirits to fill her with the "direst cruelty" so she will be cruel enough to commit the deadly act. By "crown" she means the crown or top of her head, although there is the additional imagery of a crown and overthrowing the king. This line also references gender roles. Women were seen as delicate, so she wants the spirits to "unsex" her so she can be more tough and manly. This thought is continued when she asks them to "come to my woman's breasts, and take my milk for gall." Basically, these lines are her desiring the power and strength that will be required in this conspiracy to kill the king.