Please explain the following quote from Macbeth. Who says it? And to whom?Sleep shall neither night nor day Hang upon his penthouse lid; He shall...

This is spoken by the First Witch to the other two Witches
in Act I, scene iii.  First, it should be noted that, in stage time, we see and learn
much more about the Witches in Act I than in all the rest of the play combined.  They do
have a significant scene in Act IV, but they are, at this point in the play, about
Macbeth's business and not there own, so they don't really stand alone as characters in
the same way that they do in Act I.

In this quote, the
First Witch is describing how she plans to torment a sailor, the unlucky husband of a
woman who had "chestnuts" and would not share them with the First Witch.  The First
Witch, as stated in the above quote, plans to make sure he cannot "sleep" for a number
of days, and, because of this lack of sleep (which is necessary for all humans), he will
waste away and become sicker and sicker as a result of the curse she puts on
him.

This story would have had great significance to
Shakespeare's audience in the early 17th century, for witch hunts were afoot in Europe. 
As they did in the American colonies, in Europe they created a sort of epidemic of fear
of strange behaviour and accusations of witchcraft for natural ailments and sicknesses. 
Shakespeare alludes to that here -- as a witch describes how, in retaliation for not
getting what she wanted, the husband of the woman who treated her poorly is "taken ill"
and mysteriously seems to waste away.  The witch's curse, in this case, would be
responsible for this.

Please follow the links below for a
paraphrase of this quote, plus more on this scene and witches in Elizabethan
England.