The speaker begins as if in mid-sentence. Beginning with the word "and" throws readers right into the scene in an abrupt, almost startling way that reflects the speaker's own surprise as he "stumble[s] suddenly upon the thing." The beginning of the poem, in other words, plunges readers into the scene in the same way that the speaker unwittingly finds himself staring at something unexpected in the middle of the woods.
But it's not yet clear what, exactly, this "thing" is that the speaker has found. Readers only know that this thing is in a "grassy clearing guarded by scaly oaks and elms"—that is to say, a secluded place outside the public eye. This gives the poem's atmosphere an ominous quality, as if the speaker has come across something hidden and sinister.
The fact that the speaker calls whatever he's looking at "the thing" doesn't necessarily mean he doesn't know what it is. Instead, this vague description hints that whatever he's come across is so startling that he hasn't quite wrapped his head around it yet. He's looking at something deeply troubling—so troubling, in fact, that he can't bring himself to fully recognize it or even name it in the poem itself.
Readers will later learn that the speaker has come across the scene of a lynching, where a racist mob tar-and-feathered a Black man before burning him alive. Given the shockingly gruesome nature of this crime, it makes sense that the speaker is hesitant—or even unable—to quickly come to terms with what he's seeing.
And yet, he can't keep the reality of this scene at bay for very long. Soon enough, the "sooty details of the scene" jump out at him. He suggests that these "details" cut him off from the rest of the world, "thrusting themselves between" him and everything else—basically overwhelming him so intensely that it's impossible for him to look away, keep walking, or really do anything other than take in the horrific reality of this violence. The word "thrusting" is especially telling here, as it presents racist violence as an intrusive, unrelenting force, something that essentially assaults the unprepared speaker.
There's quite a bit of alliteration and assonance in these opening lines. Note, for example, all the /w/, /s/, /g/, /uh/, and /ee/ sounds:
And one morning while in the woods I stumbled suddenly upon the thing,
Stumbled upon it in a grassy clearing guarded by scaly oaks and elms.
And the sooty details of the scene rose, thrusting themselves between the world and me...
These devices add emphasis to the speaker's words, elevating the language in a way that immediately snaps readers to attention. This heightened poetic sound thus draws readers into the poem, pushing them into the scene with the same kind of abruptness that the speaker himself experiences when he suddenly stumbles upon the corpse.