What is Macbeth talking about in this quote, how does this quote relate to the plot? and what is the significance of this quote?Who can be wise,...

The killing of the grooms is the proverbial beginning of
the end for Macbeth.  It's important for numerous
reasons. 

It's the first time Macbeth veers from his wife's
plan.  From this point on, concerning the plot, Macbeth will make his own decisions,
without even consulting his wife.  He will shut her out of his decision making.  This is
a mistake.  His wife plans well and sees the big picture, or the consequences of an
action.  Macbeth does not.  When he arranges the murder of Banquo and Fleance, he leaves
little doubt that he is guilty of treachery, and he becomes a suspect.  His killing the
grooms leads immediately to suspicion on the part of Macduff, and Macduff is, of course,
the hero of the play who will bring Macbeth
down. 

Incidentally, while Lady Macbeth's feinting may,
indeed, be a stunt to divert attention from her husband's mistake, it may also be an
actual incident of feinting:  feinting caused by her husband's idiocy at being so stupid
as to kill the grooms.  She is shocked at his actions.  The grooms would have had no
information to give except that they were drunk and asleep.  That isn't much of an alibi
when a king has been killed. 

In the quote you ask about
Macbeth also provides another example of what appears fair really being foul, thereby
furthering that theme.  He explains how reason left him and emotion, his love for
Duncan, took over.  He covers himself by saying that any man who really loved his king
could do no other than kill those who killed him.  Interestingly, notice how Macbeth
indirectly calls himself a hero, here.  He not only evades any condemnation for killing
the grooms, but makes himself out a loyal Thane and follower of Duncan for having done
so.  What great Thane wouldn't avenge his king?  He thereby claims to love Duncan more
than the others--for they controlled their love, while he could not--his love is
"violent" love. 

Macbeth claims that he couldn't help
himself, that reason left him.  Duncan's skin is "silver" and his blood "golden."  The
dagger wounds are like some great breach in nature.  The real breach is the unnatural
act Macbeth has comitted--which, by the way, contributes to another theme.  That of the
unnatural.