Whitman sees a hawk and feels humbled. In his mind, the hawk says, "Dude, you've been talking, like, forever. Get this poem over with and quit yer' daydreamin'!"
Whitman sees himself in the hawk. His voice is "untranslatable" and, in another famous phrase, a "barbaric yawp." (A "yawp" is like a brute, animal sound and not a part of a refined language. It has elemental power.)
The day seems to wait for him to get ready to move on. It leads him on into darkness.
The ending day might be a metaphor for death. At any rate, the poet's hair has grown "white," and he shakes his "locks" at the setting sun. He dissolves into the air, leaving like the air and fusing his flesh in the "eddies" of water. He gives himself up to the dirt.
This imagery ties together the earlier idea of the grass containing the bodies of dead people. If we want to find Whitman, we have to look at the ground under our boots.
When we find him, we won't have any idea who he is, but he'll work his power on us anyway. He gives good health to people who walk over him.
Strength and good health! We've got to go searching for this Whitman fellow.
Whitman ends the poem by saying that we shouldn't be discouraged if we go looking for him but can't find him. If he's not in one place, we should search in another.
He's not running away from us or trying to avoid us. He has stopped ahead of us on the journey. He's waiting for us to catch up.