The adage, "Those who fail to learn from their pasts are bound to repeat them," is one way of thinking about the interpretation of this famous passage. Another adage also can be applied here, that is, "Repeating the same thing over and over and expecting different results is the definition of insanity."
The world the characters live in is "old" but they do not learn from their mistakes. They are so unwilling or unable to examine their pasts in a clear light that the events they have lived through seem "unknown." They keep on trying to re-live the past, in a deluded and nostalgic way thinking the past was pure happiness, but also, simultaneously and contradictorily, trying to overcome it. This duality and conflicting desire is apparent in the earlier portion of this quote, "tomorrow we will run faster, stretch out our arms farther . . .” They find themselves, then, rowing against the current and staying in virtually the same place.
The green light is a symbol of envy and jealousy but also of the unattainable. This light can be applied not only to the desires of Gatsby but to the American dream itself. It always seems so close yet just beyond reach, promising fulfillment but delivering frustration.