This poem is an extended metaphor and rain is personified. It is much easier to understand if you catch from the very beginning that the speaker (the voice of the poem) is the rain itself.
I urge you to re-read through each stanza of the poem keeping this in mind. Generally speaking, the entire poem sounds like a poetic autobiography of "rain." Notice the mention of natural elements that are commonly associated with rain: fields, hills, flowers, cloud, thunder, rainbows. This poem explains these elements as though they are each in a unique relationship with the rain. Notice also the images like: emerging from the sea, touching windows, killing heat, heaven crying. These images suggest action and movement, which of course, perfectly embodies rain.
This poem does not tell a story, but rather, relies on nature imagery and delicate diction to present the voice of rain. Even the short stanzas and visual structure of the poem suggest little rain droplets, moving, washing, healing, and singing. It could be a tribute to nature or simply an appreciation for something otherwise so common that it is often ignored.