The Latin word ars is translated variously as stratagem, technique, art, craft. MacLeish's poem title "Ars Poetica" can be translated loosely as "The Art of the Poet." This is a strangely contradictory poem because even as MacLeish claims that a poem should be silent ("dumb") and "palpable" (tangible, plainly touched) and equal to no meaning ("Not true"; "not mean"), he is clearly presenting a poem that means a great deal: It presents his poetic aesthetics, his art of poetics, and his philosophy of the definitive nature of poetry. So, while his couplet metaphors and similes are gracefully poetic as well as ironically contradictory, they belie his message though they don't negate the obviousness and the import of his message. ("Curiouser and curiouser!" cried Alice.) Perhaps he means to imply that a true poem lies in the majesty of nature and that a written poem is only an inept imitation.
Consideration of one couplet, "A poem should be wordless / As the flight of birds," points out the obvious contradiction and, incidentally, supports the idea of the theme of a true poem of nature and an imitation poem of words. By virtue of the nature of written language, a poem read by readers must contain words. This ironically contradictory couplet may relate to his art of poetry in that it suggests he takes his poetic inspiration from that which is wordless, such as "the flight of birds." In that case, in support of the idea of true and imitative poems, it would be correct to say that a true poem is, in his philosophy of poetic aesthetics, found in nature.
Another couplet is also contradictory but more easily applied to the art of poetics: "A poem should be motionless in time / As the moon climbs." The ironic contradiction lies in the fact that the moon climbing in the horizon is not "motionless in time" as observed by one viewer. However if considered over a vast time, for example all of poetic history, then the moon might metaphorically be considered "motionless" because it has climbed the horizon in all eras, from the time of the poets who spoke "The Wanderer" to the time of Spenser's Amoretti to MacLeish's time, which ties in with the couplet's application to poetics. A poem is meant to be an immortal thing, as has been agreed by poets since time immortal, that brings poetic revelation to all ages such that it is "motionless in time."
Incidentally, the Aristotelian poetic, also described by Sir Philip Sydney, is that a poem is a divinely inspired imitation of God's spiritual truth that the hearts of humans desire to know but can't attain except through poetic inspiration. This parallels the idea of a true poem of nature and an imitative poem of words suggested by MacLeish's poem. The difference being that for earlier poets the inspiration and true poem was God's spiritual truth while for MacLeish it is nature.