Ars Poetica, or The Art of Poetry, was a book written by Homer in 18BC. This title has been used repeatedly, or referred to repeatedly, over time. Homer's book referred to many different ideas surrounding the writing of poetry. Outside of Homer's book of poetry, Archibald MacLeish wrote a poem named "Ars Poetica" in 1926 (many, many years after the original).
MacLeish's poem is not nearly as complicated as Homer's poetic ideology (naming of poetic methods such as "in media res" (later adapted by the writers of epic poems such as Beowulf), "bonus dormitat Homerus" (which refers to the fact that all poets, even the skilled ones, can err in their poetry), and "ut pictura poesis"(which described the careful attention to poetry--the same as great artists used in sculptures and paintings)).
That being said, MacLeish's poem is just as importantly a work of art. The poem begins "in media res," or "in the middle of things." It starts describing what a poem should be without leading up to what a poem is.
Secondly, the poem brings up the ugliness of a poem. By using words such as "dumb," "old," and "moss," the poem does not necessarily paint a beautiful picture of poetry. Here, MacLeish is noting the "bonus dormitat Homerus," or the errors in poetry.
Lastly, MacLeish details throughout the poem the images which can be associated with all poetry. By doing this, he is noting the "ut pictura poesis," or the detail with which poets need to pay attention to their works.
In the end, MacLeish uses Homer's idea of poetry (the nature of poetry) to create a poem in its own right.