The green light symbolizes Gatsby’s hope and downfall as it relates to Daisy. It also represents the potential for a new beginning, hope, aspiration, and the pursuit of the American dream. As well as its downfall, as we pursue something that is not obtainable, as time goes on.
Here’s F. Scott Fitzgerald’s summary from the last chapter when Nick says:
“…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 such a long way, 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 beyond the city, where the dark fields of the republic rolled on under the night.
Gatsby believed in the green light, the orgastic future that year by year recedes before us. It eluded us then, but that’s no matter. Tomorrow we will run faster, stretch out our arms farther until one fine morning.
So we beat on, boats against the current, borne back ceaselessly into the past.”
The Great Gatsby by F. Scott Fitzgerald’s (1925)