What exactly is a metaphor?Metaphors are pretty simple to understand. You compare one idea or concept to another idea/concept. Then, you explain the link between the two. An easy example is: "You filthy pig! Clean your room. You do lay around in your slop and make not an oink to fix it!" Here, the person being spoken to is being compared to a filthy pig. Likewise, the person's room is being compared to a trough of slop (mud). Lastly, the fact that the person will do nothing is compared to not making 'an oink'.
So what's the big deal?
The unique thing about metaphors is that everybody likes metaphors because metaphors are the way the human brain thinks. People make connections to new experiences by comparing and relating them to past experiences in a symbolic way. Therefore, a particularly good metaphor allows the reader to tie in many known experiences to better understand exactly how the author is feeling. After all, no one will how the author feels better than the author (duh.) But, an author can make his/her experiences more real to the reader by including vividly symbolic metaphors that will help the reader imagine the situation more clearly.
And that is the purpose of language in itself: To make our experiences more real to others and vice versa.
One last example
Say, for instance, in The Kite Runner, Khaled Hosseini wants to portray how the character Amir feels that he and his father, Baba, do not have a strong father-son bond, but flying kites is one activity that they both enjoy.
One way to put this is:
"Baba and I live together, but flying kites is the only thing we like to do together."
Whereas Hosseini puts it:
"Baba and I lived in the same house, but in different spheres of existence. Kites were the one paper-thin slice of intersection between those spheres (Hosseini,52)."
As you can see, the difference is as wide and noticeable as the horizon of the Caribbean Sea. =)