Of course "Ars Poetica" (Art of Poetry) is about art. What were you expecting it to be about? The life cycle of the mealworm? But it's also about how to experience art and, in some ways, life in general. For a moment we get to feel out of ourselves, no matter if the speaker is talking about poetry or birds flying in the sky. Any way you cut it, we get to go beyond the physical world and explore the metaphysical world of art and poetry.
Bottom line: poetry is about life. In MacLeish's poem, then, poetry must therefore resist defining something as indefinable as life.
Relax, everybody. Art and poetry shouldn't try so hard to prove something; rather, it should be just as free as life itself.
It's a modern poem, so you know "Ars Poetica" is likely going to delve into the tricky world of reality and our frequent, but futile, attempts to define the world around us. And yet, MacLeish keeps things relatively simple for us here. Reality, in his poem, is about "being" rather than "meaning." So put away that calculator and dust off your imagination already.
Reality, in MacLeish's poem, is a matter of perspective and "being," rather than a concrete depiction of the world around us. Trippy, we know.
Time for a cliché: a picture says a thousand words. If we're going to try to define reality in MacLeish's poem, it makes sense to do so via imagery, rather than so-called "truths."
Even though a poem should be "wordless," according to our speaker, "Ars Poetica" still has words in it. (We went back and checked, just to be sure.) And since the speaker is talking about poetry and the ideas a poem might communicate, it's safe to say that we have some questions floating around concerning language and communication.
Language is a tricky thing when it comes to good poetry; it should make us feel something but it shouldn't scream at us at the same time. AT THE VERY LEAST AVOID ALL CAPS.
Words can never really communicate truths to us, whether in poetry or anywhere else, but they can make us feel "truth" as we perceive it. Far out.