What does this Great Gatsby quote mean?

And as I sat there, brooding on the old, unknown world, I thought of Gatsby's wonder when he first picked out Daisy's light at the end of his dock. He had come such a long way to this blue lawn, and his dream must have seemed so close he could hardly fail to grasp it. "But what he did not know was that it was already behind him, somewhere in the vast obscurity beyond the city, where the dark fields of the republic rolled on under the night."