Throughout the entire novel, the green light serves as a symbol of hope and the American Dream -- specifically, Gatsby's dream to attain Daisy and her affections. Literally, the green light is, in fact, a green light on the end of the Buchanans' dock, and Gatsby can see it from his house across the bay. We are first introduced to Gatsby (or at least his dark figure by the shore at night) and the green light at the end of Chapter 1 of the novel, when Nick notices Gatsby
[...] stretch[es] out his arms toward the dark water in a curious way [...] [then Nick] distinguish[es] nothing except a single green light, minute and far away, that might have been the end of a dock (Fitzgerald 21).
As the novel progresses and Gatsby begins to grow closer to Daisy, he tries to speed the process of reuniting with Daisy. It becomes apparent that Gatsby has bought his house on West Egg precisely to be close to Daisy, and he becomes determined to win her affection and to secure his own hopeful fate no matter what else may stand in his way. Even a note of desperation emerges when he tries to casually mention later at the Buchanans' house,
You always have a green light that burns all night at the end of your dock (Fitzgerald 94).
Why mention this seemingly innocuous, yet well-known fact, unless he were trying to draw attention to the fact that he stares across the bay and recognizes that green light as a symbol of where Daisy is? Gatsby's desperation for Daisy also manifests itself in his blatant statement to Nick,
Can't repeat the past? [...] Why of course you can! (Fitzgerald 110).
For Gatsby, the green light -- and Daisy -- are symbols of a future he is trying to attain, a future that he ultimately desires will mimic the past.
Finally, at the end of the novel and after Wilson literally kills Gatsby's dream, Nick ponders over
Gatsby's wonder when he first picked out the green light at the end of Daisy's dock [...] his dream must have seemed so close that he could hardly fail to grasp it. He did not know that it was already behind him (Fitzgerald 180).
The sad truth, at the end, is that Gatsby never could have relived his past with Daisy and it was his drive to create his own happy fate that actually destroyed him. Whereas green usually means "go," in this case, Gatsby should have stopped before he began in his quest for Daisy. She was never meant to be his American Dream, and by pursuing her, he actually contributed to his own demise.