He addresses death and says, "What, you think I'm afraid of you?"
He compares death to a kind of birth. He watches someone's death from the "door" of life and watches the relief of someone passing through it.
He then expresses an idea that might make some people uncomfortable. People who have died and who have returned to the ground (been buried) make good fertilizer that leads to the growth of new life. He has no problem with this idea.
At any rate, he has been through death thousands of times before.
Remember that earlier in the poem he described the grass as growing out of the graves of dead people. He now revisits the same idea and repeats, "O grass of graves."
He describes the moon shining on an "autumn forest" and then raises himself to look at the moonbeams.