What is the poem saying in "Ars Poetica" by Archibald MacLeish?

Archibald MacLeish's poem, "Ars Poetica" is about the art of poetry.  In it MacLeish leans heavily toward the idea of concrete imagery.  Each of the 12 couplets that comprise this poem give a simple, concrete definition of poetry that pit images against common things such as stone worn down by many people (sleeves) having rubbed against it or an open, abandoned doorway littered with fallen leaves.  The idea is that these images are clear in the mind of the reader, as clear as they were in the mind of the poet, thus the poetry has succeeded in giving the reader the same feelings the poet had when he wrote the poem.  MacLeish feels that a poem's job is to do just that - recreate in the mind of the reader the same image the poet had as he created the poem.  In that regard the poem could be viewed as an instruction on how to write a poem.