Before Gatsby was reunited with Daisy, he spent significant amounts of time and energy dreaming of the day when he would be with her again. As he built up hopes and fantasies, he often spent time gazing into the distance - searching for the connection that would bring her back into his life, imagining how the forces in the universe could end their separation.
Standing on the porch of his mansion and looking across the water separating West Egg from East Egg, Gatsby had been aware of a green light across the water. Having learned that the light was located on the dock belonging to the house where Daisy lived, Gatsby is amazed, almost envious, at the proximity of that light to the object of his longings for all that time.
Compared to the great distance that had separated him from Daisy it had seemed very near to her, almost touching her. It had seemed as close as a star to the moon.
Now, however, there no longer was any special meaning associated with the light. Daisy was back in his life, standing next to him, and the light - no longer his only connection with Daisy - could return to being merely "a green light on a dock."
This quote is from chapter five in F. Scott Fitzgerald's novel, The Great Gatsby. The full context of the quote is listed below:
"Possibly it had occurred to him that the colossal significance of that light had now vanished forever. Now it was again a green light on a dock. His count of enchanted objects had diminished by one"
Gatsby used to watch across the water from his mansion in West Egg to where Daisy lived in East Egg. He desires above all else to win the love of Daisy, whom he met when he was a military officer before he went to fight in World War I. For Gatsby, the green light on the dock symbolizes the hope he has that he will be reunited in love with Daisy forever. It represents his one dream and aspiration.
Through Nick Carraway, Gatsby is reunited with Daisy and, because she is unhappy with her husband's extramarital affairs, she begins a relationship with Gatsby. Once Gatsby has achieved his heart's desire in being united with Daisy in a relationship, his hopes give way to reality, and all that the green light symbolized is his now. He no longer needs the beacon of hope that was the green light on the dock. That is why his count of enchanted objects has diminished by one.