what is the green light that Gatsby is starting at

The green light at the end of the dock is a symbol of Daisy because it is her dock.  She is somewhat a symbol of Gatsby's hopes and dreams, is go ahead for his future.  More than that, a green light shows how much he wants to achieve:  Daisy, success, a future that is everything he hopes. He is almost symbolic of the American who also has such hopes for the future.