What does the green light seem to symbolize at the end of the book?

The green light in The Great Gatsby is symbolic throughout the book. When Nick sees Jay Gatsby on Daisy Buchanan's dock, it appears to him that Gatsby is reaching for the light as if it is his future, lighting his way to the American Dream. That the light is green serves as encouragement to Gatsby, giving him the go-ahead to pursue his dream, much like a green traffic light signals "go."
Green is also symbolic of something new, as in the tender buds of plants, the budding green leaves of trees, and the burst-open flowers as sunshine warms the earth in the spring. Green is young and new, which signifies Gatsby's dream for a new life with Daisy, as she is a 'flower' and Gatsby equates her to youth.
At the end of Chapter 9, Gatsby has realized that Daisy is not young and innocent, but a grown woman who is indecisive and morally flawed. Gatsby realizes he will never fulfill his dream of a love-filled life in the upper echelon of New York society with Daisy on his arm, signaling his financial and personal wealth. Gatsby's pursuit of the American Dream he so badly desires has ended. Gatsby's realization coincides with mist coming in from the sea. As the green light disappears in the gathering gloom, so does Gatsby's dream fade from sight. As chapter 9 ends, we get confirmation that we cannot change our futures: "So we beat on, boats against the current, borne back ceaselessly into the past."

The green light at the end of The Great Gatsby may be seen as symbolizing Gatsby's desire to marry Daisy to have her because he loves her and because he believes that obtaining her would give him the social status he so badly desires--the status that money and audacious social events cannot alone give him. It represents his desire to advance in society, much like a green light advances traffic. That the light is blinking means it is not an easy task, as he can only see it half the time. We as readers understand that Gatsby will not realize his dream when Fitzgerald says of Nick Carraway when he first sees Gatsby: "I glanced seaward – and distinguished nothing except a single green light, minute and far away…”. The presence of the green blinking light at the beginning and the end of the book represents the circle of life and death.

In Chapter Nine, as Fitzgerald closes the story, he leaves the reader with the image of a green light, the light which was first introduced in Chapter One as a symbol of Gatsby's desire to win back Daisy:

Gatsby believed in the green light, the orgastic future that year by year recedes before us.

By Chapter Nine, however, this light has become symbolic of Gatsby's inability to attain his dream. As we see through the story, despite his numerous efforts, Gatsby was never able to convince Daisy to leave her husband, Tom, and spend the rest of her life with Gatsby instead. Moreover, Gatsby's death ensured that he would never get another chance to try and win her back. The green light therefore represents the shattered hopes and dreams that Gatsby once possessed.
The green light is also symbolic of Daisy herself. For Gatsby, Daisy has shone just as brightly in his mind as any real light. She literally brightened up his life and made him determined to win her back.