"A poem should not mean / But be."
MacLeish seems to believe that a powerful poem's meaning will wash over the reader and have its effect without the reader necessarily knowing what the poem "mean[s]" in the literal or formal sense. Modernist poetry sought to experiment with meaning and form to accomplish just that: work that would move the reader in ways not decipherable through direct analysis or understanding. Put simply, a poem should be too much for words to explain, instead engaging and acting within the realm of sensation, imagery, being and emotion.
"A poem should be wordless / As the flight of birds."
The author is trying to explain that a poem should be so evocative, natural and pure, like the flight of a bird, that the reader almost forgets they are reading a formal construction of words. A poem should be active in the mind's eye, not limited to the technical meanings of the words. The concept of "the flight of birds" is important, because birds do not think about how to fly, or whether there is any other way to exist besides flying. Much like the flight of birds that just is, a good poem should function outside of construction, formality and cognition.
"A poem should be motionless in time"
This phrase is complex, because on the one hand the speaker could be indicating that a poem should be stalwart in the face of time, therefore transcending its movements and remaining relevant or meaningful for eternity. One the other hand, the speaker could be indicating that a poem should be precisely of its moment—not looking backward or forward, but existing in a hyper-present manner (motionless precisely in or through time). Part of this quality might be the poem's ability to keep the reader lost in the impressions and evocations of the poem, momentarily forgetting his context in space and time.
