What is the significance of the green light?

Because the green light is at the end of Daisy's dock, it is obviously related to Gatsby and his dream of pursuing and winning Daisy back into his life and his arms. It is guiding him through the mists of the night to the place where his love resides; in general, most people think of a green light as a signal for "go" and certainly Gatsby wants to go and take up where he and Daisy left off so many years ago. Nick even makes a reference that it might have been a signal for the early settlers who saw the green of the land when they first came to this part of the country.