If I am recalling Easterbrook correctly, this quote, which I have seen all over Wikipedia and the internet. Is kind of misleading. In the source article, "Somehow Distrubed at the Core: Words and Things in William Carlos Williams" Easterbrook is addressing early one about the rigid inflexibility of how man early scholar's approached Willaims and thus did not understand the flexibility and depth of Williams' work. When he speak of the "Red Wheelbarrow" in this bit of literary criticism, he is talking about the way in which Williams frames his work through his title, much like how a picture frame can give focus and depth to a painting. Willaims had a sharp artistic eye, and many of his poems, particularly his ekphrastic collection Pictures from Brueghel were inspired as "paintings in words". What Easterbrook was demonstrating was that altering the title, his example was to XII, alters the "still life" quality of the poem, and would also take way how Willaims saw beauty in the mundane. A simple title to reflect simple beauty.
So, to make a short answer long, Easterbrook is pointing out how a poem's title frames how a reader will begin to look at the poem, and develop their personal meaning/relationship with the poem. Try to image a photgraph of you and a friend just smiling. Now were I to put that photograph into brightly colored frame with maybe a goofy saying or a fun shape, that photo would feel much different then if I put it into say a formal black frame. One feels like a good time, one almost feels like a picture would would hang in remembrance, perhaps at a funeral or memory of a loved one. When Easterbrook say "frame" this is basically what he means. Choosing a different title woudl mean altering the way we react and find meaning in Williams' poem.