In The Great Gatsby, what does the green light at the end of the Buchanan's pier's symbolize?

That light symbolizes all of Gatsby's hopes and dreams. 
On the very second page of the book, Nick says of
Gatsby,

...it
was an extraordinary gift for hope, a romantic readiness such as I have never found in
any other person...(6).

Of
course, Gatsby's yearning is for Daisy, who is herself a symbol of all that Gatsby
aspires to, the American dream, the idea that everyone can succeed here, no matter how
humble the beginnings, by working, wishing, and hoping.  Everything that Gatsby has done
since he met Daisy has been with the aim of acquiring her.  He believes that if he does
so, he will have achieved the American dream.  Of course, given who Daisy is, that
dream, had he achieved it, would have been an empty one, and his dying might be the more
merciful end for him, rather than his having lived and learned that his dream was rotten
to the core.