Macbeth Quote Significance

What is the significance of the quote:

"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 th’access and passage to remorse,
That no compunctious visitings of nature
Shake my fell purpose, nor keep peace between
Th’ effect and it. Come to my woman’s breasts,
And take my milk for gall, you murd’ring 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!’"