I believe the poem is titled, "The Voice of The Rain." In the poem, the rain is personified and the speaker of the poem asks it directly to explain itself.
The rain responds by talking of how it is pure and intangible to normal perception. It goes on to talk about how it follows a cycle where it descends from the heavens, comes to earth, and then rises again. Here it takes a very spiritual feel coinciding with images of a god who falls to earth and then rises again. Most common to Western cultures would be the story of Jesus' death and Resurrection. The rain further pushes this idea by talking how everything on the earth is but seed and dust until if falls and things are able to grow and bloom.
In the end, Whitman is connecting the sublime with the everyday of falling rain, and his spiritually with nature, ideas common with Romantics.
The complete title of this poem is "The Voice of the Rain". Written by Whitman in 1885 as part of his anthology of poems Leaves of Grass, the poem is a conversation between the author or narrator and the rain.
Simply, the author/narrator upon experiencing a rain shower stops to ask the shower who it is. The rain proceeds to answer in descriptive language, explaining who and what it is and how it works. Though the language is poetic, the reader can easily pick out the parts of the water cycle so the poem quite literally describes rain.
On a deeper level, the rain answers that it is the "Poem of the Earth" and describes itself in a way that compares it to poetry. It describes how the source of the rain like poetry, can rise from different sources, be transformed and reborn, providing life to the receiver.