What's the deal with Gatsby's green light?

Briefly: the green light symbolizes Gatsby’s desire — primarily for Daisy, and more generally for an idealized past he’s determined to recover. It’s the distant, hopeful object that drives his reinvention and extravagant behavior, but by the end Nick sees it as emblematic of the larger American dream: bright with promise yet ultimately unattainable and emblematic of disillusionment after Gatsby’s death.