The poem begins by describing the young farmhand of the title attending a dance—or, more accurately, standing outside the dance hall and pretending not to care about said dance!
The poem immediately distances the farmhand from everyone else by turning the reader into a direct observer of the scene: "You will see him," the speaker begins. The reader is thus aligned with the speaker, set apart from the young man and instead sharing in the speaker's observations.
These four lines show the young farmhand trying to appear cool and detached. He lights his cigarette in a "careless" way. He leans against a wall, tells jokes, and looks off into the middle distance like some kind of rural James Dean.
But the poem signals to the reader that, for all his outward projection of coolness, on the inside he's anxious and angsty. The farmhand's actions are presented as a kind of list, using both asyndeton ("At the hall door careless, leaning his back / Against the wall") and polysyndeton ("or telling some new joke / To a friend, or looking") to give the impression that all these actions are again somewhat studied and self-conscious. It feels like the farmhand is going through the motions of what he thinks he's supposed to do to seem like he's not interested in what's going on inside the dance hall.
A number of other devices here represent the farmhand's attempt to seem aloof. Lines 1-3 are all enjambed, for example, creating a kind of laid-back flow to the farmhand's actions.
The stanza is also full of soft /l/ sounds, in "will" and "light" (line 1), "hall," "careless," and "leaning" (line 2), "wall" and "telling" in line 3, and "looking" in line 4. This consonance has a languid gentleness to it that mirrors the farmhand's attempt at looking cool and unhurried.
Finally, it's worth reiterating that the farmhand is "at the hall door," rather than properly inside the dance hall itself. This paints him as a kind of outsider and a peripheral figure, as though some kind of force field—perhaps his status as a farmhand—prevents him from ever fully entering the perhaps more middle-class and modern world represented by the dance.