Another way to ask this question would be: What isn't "Song of Myself" about?! Walt Whitman's long poem, with its 52 sections of open verse, is about just about everything under the sun.
Take the opening section for example. It begins talking about an "I," the speaker (who in a number of ways resembles the poet himself) and "you" (whom I've always taken to be the reader or listener of the poem). The speaker then quickly moves on to talk about his soul and spears of summer grass, the smells in the air, his breath, the pleasures of the body and of nature and of the city, all sorts of human experiences and occupations, and on an on. In the end, I suppose it's not easy to answer in just a sentence or two what this poem is about.
My best attempt would be to say that "Song of Myself" is about affirmation (finding "good" all over the place) and about the beauty and delight of both the human body and the human soul. To me, it's always meant a lot that Allen Ginsberg singled out one line in Whitman's poem for particular praise: "I find no sweeter fat than sticks to my own bones." He said that the meaning of the line really hit him and stuck with him when he heard his fleshy high school English teacher read the line aloud.