Did you know that Faulkner considered himself a "failed poet"? (source). His first book was a book of poetry called The Marble Faun. You can read a review of it here. Influential critic Cleanth Brooks says that Faulkner "discover[ed] that his richest and most fully formed writing – his poetry in short – could be realized in his prose" (source: "Faulkner as Poet." The Southern Literary Messenger 1.1, 1968, p. 5). Prose simply means writing that isn't poetry (i.e., writing that isn't in verse form). What Brooks suggests is that Faulkner was able to poeticize his prose. His poetic style allows him to hit the emotional and sensory high notes of poetry without giving up the character development and story lines of fiction.
An important characteristic of poetry is its ability to evoke emotion and convey meaning in a small space. Think how much meaning and emotion is packed into the simple lines, "Father! Father!" (107). Another quality often found in poetry is ambiguity. An ambiguous line is open to multiple meanings. This is true of many lines in "Barn Burning," including the one we just quoted. Sarty's exclamation includes sadness, loss, and probably relief as well. Multiple meanings give the reader more freedom to interpret, while mirroring the mystery and complexity of real life.
Now, some readers criticize Faulkner's style for being confusing and hard to read. The super long sentences are hard to navigate. In poetry, the poem is supposed to grip the reader, and even carry the reader away. This is what Faulkner's long sentences can do. He seems to unleash them at moments in his stories where emotion and action have reached a peak, like when Sarty is running after he warned de Spain.
We are supposed to feel as lost, confused, and carried away as Sarty. Even if we try to slow down, the sentence moves us along. Similarly, even though Sarty might want to slow down and get a grip on the situation, the events of his life keep him on the move. So, our best advice is to go with the flow of the story. Also: trust your intuition; look up words you don't know; and read confusing moments out loud. As with poetry, nuances of "Barn Burning" are revealed when read aloud.
Now, for a particularly poetic example:
He went on down the hill, toward the dark woods within which the liquid silver voices of the birds called unceasingly – the rapid and urgent beating of the urgent and quiring [choiring] heart of the late spring night. He did not look back. (109)
Here the narrator likens the singing of the birds to the beating of a heart, specifically, the "heart of the late spring night" to produce a dramatic characterization of the woods Sarty seems about to enter. Because this idea is ambiguous, condensed, and difficult to grasp with our powers of rationality, we have to let our senses and intuition take over.