We were all set to break this down for you Shmoopers, but hey—Robert Penn Warren wrote his own summary:
"The scene is a broad meadow with a number of the enormous live oaks of Louisiana strung with grey Spanish moss. The people involved are two lovers who have been lying in the shade of one of these trees after the sun goes down. The shadows of the trees now tend to level out as though they were water, as though the lovers were submerged in the water. They compare themselves to a coral growth that has been submerged for thousands of years, as they feel submerged by the human history before them. Though all of us are trapped in time, and live in time, so little time, there are moments, say in love, that seem outside of time—moments in which 'we practice for eternity.' I can't do more to explain it."
You heard the man. Lucky for you, though, we can do a whole lot more to explain it, so read on.