The green light at the end of Daisy's dock is one of the most obvious symbols in The Great Gatsby by F. Scott Fitzgerald. The green light, which Gatsby can barely see from the end of his own dock, represents the hope (dream) Gatsby has of being reunited with Daisy, the woman he loves.
The green light is only mentioned three times in the novel. The first night the narrator, Nick Carraway, meets his rich and elusive neighbor, Gatsby stands silently and then reaches out his arms toward something unattainable across the water. Nick says:
[H]e stretched out his arms toward the dark water in a curious way, and far as I was from him I could have sworn he was trembling. Involuntarily I glanced seaward--and distinguished nothing except a single green light, minute and far away, that might have been the end of a dock. When I looked once more for Gatsby he had vanished, and I was alone again in the unquiet darkness.
The next time the green light appears is in chapter five, when Gatsby mentions it to Daisy.
"If it wasn't for the mist we could see your home across the bay," said Gatsby. "You always have a green light that burns all night at the end of your dock." Daisy put her arm through his abruptly but he seemed absorbed in what he had just said. Possibly it had occurred to him that the colossal significance of that light had now vanished.
The significance of the light is lost on Daisy, who barely even realizes she has a green light at the end of her dock. Gatsby, on the other hand, seems to understand that there is no longer a need to see the light as a symbol because he has the reality with him now.
Finally, in chapter nine, Nick reflects on that green light which must have been one of the only things sustaining Gatsby's hope for so many years. Nick says,
And as I sat there brooding on the old, unknown world, I thought of Gatsby's wonder when he first picked out the green light at the end of Daisy's dock. He had come a long way to this blue lawn and 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, somewhere back in that vast obscurity.
Clearly the green light is at first a symbol for Gatsby of his hope for a love he had lost but wants to recover; as soon as that hope is realized, the symbolic nature of the light is gone, and it becomes just a simple green light at the end of a dock.