In her poem, Moore is saying that we can dismiss poetry as so much fiddle (nonsense) and that what passes for poetry can, in fact, be that. However, she says, when a poem communicates something genuine, it can be useful, just as "'business documents'" can be. However, poetry is not the same as schoolbooks and business documents because ofits usefulness arises from its ability to evoke emotions, such as when we feel real fear:

eyes that can dilate, hair that can rise

Real poems are important not because they can be interpreted in arcane ways by critics but because the emotions they communicate are genuine. When emotions are genuinely conveyed, using the raw material of specific descriptive detail, such as a sense that there are "real toads" in the garden, then real poetry can emerge.

Moore is saying that real poets convey authentic emotions or emotional experiences using fresh ("raw"), concrete language. She differentiates between this and what is produced by "half poets" who can sound poetic but are, in reality, arrogant and trivial.

Admittedly, this definition doesn't get us too far, but it does leave it to the reader to determine if the images and ideas conveyed in a poem are raw and real enough to be emotionally moving and memorable—at which point, Moore asserts, you are reading a true poem.

Marianne Moore's poem "Poetry" is a commentary on the power of all words. The speaker urges her readers not "to discriminate against 'business documents and school-books'; all these phenomena are important." Something meaningful can be gleaned from any form of written text—if we are willing to accept it for what it is rather than trying to place "high-sounding interpretation" upon it, causing the meaning of the text to become "unintelligible."

The speaker adds that poetry, specifically, has the power to help its readers feel "raw" emotions when we allow it to. If we give ourselves over to the "imagination" that poetry cultivates, we can experience a visceral response, causing our hands to "grasp," our eyes to "dilate," and our hair to "rise." This visceral response allows us to feel a "genuine" connection to the text. She argues that anyone who enjoys this "rawness," in fact, enjoys poetry—whether they would like to admit it or not.

In her interesting dialectical poem "Poetry," arranged in five stanzas like an essay, Marianne Moore declares first that she dislikes poetry, but it is really "all this fiddle" about poetry for which Moore has a distaste.  Poetry has a place "for the genuine" she declares in her thesis statement of the first stanza. 

With a tone that is both argumentative and witty, Moore continues her dialectic in which she points out that "half-poets" who overanalyze things and try to force meanings upon phenomena are the ones that render poetry ingenuine. For, it is only when the poet creates "imaginary gardens with real toads in them" that poetry truly exists.  The combination of the imaginary with reality is what constitutes true poetry:

In the meantime, if you demand on the one hand,
   the raw material of poetry in
      all its rawness and
      that which is on the other hand
         genuine, you are interested in poetry.

When poetry focuses on the genuine, and moves away from "all this fiddle," real meaning is conveyed. Thus, poetic devices such as allusions are used not to be unintelligible, but to reconfigure truth and provide the "literal of the imagination" that is above triviality.