Have passed I thought a Whip Lash Unbraiding in the Sun
The speaker describes a state of confusion at first. At "Noon" (12) (and, remember, as a boy), the speaker passed what he "thought" to be a whip lash, or an actual whip itself. (By putting a space between "whip" and "lash," the speaker clarifies that it's not "whiplash"—an actual strike or movement of a whip—that he's talking about.)
So, what about this mistaken lash? Well, Dickinson gives us this great verb here: "Unbraiding." In other words, what the boy notices is that this "whip" looks like it has been out there so long that it’s starting to come apart.
Okay, so that's the story that this boy is telling us, but we just have to stop for a second a) to admire Dickinson's word choice ("Unbraiding" = awesome creativity) and b) wonder a bit about what's behind that choice.
The image of a whip unbraiding is a kind of unraveling mess, a once-unified whole coming apart. In this moment, the speaker is describing a mistaken perception (we, the readers, know that this "Whip Lash" is really a snake). And that confusion is really nicely reflected, we think, in the image of a whip that is starting to come apart. An unraveling whip is a bit like the speaker's understanding of his surroundings, which is about to come apart with this mistaken identification.
Of course, if we wanted to go even deeper here, we could also extend this relationship to the reader of the poem, whose prior assumption of Dickinson-as-the-speaker (i.e., a "she") has been unraveled by the reveal in line 11 that the speaker is a "he."
When stooping to secure it It wrinkled And was gone –
So, the speaker goes to pick up what he thinks is a whip then, poof! It's gone.
We think it's telling here that the speaker never says outright that he realized that the lash was actually a snake. "It" is used to describe, in our mind as readers, the snake, but in the actual syntax of the poem, it refers to the whip itself. It's like the whip suddenly became animated and slithered ("wrinkled") away. And giving an inanimate whip the ability to crawl away would be another example of personification. That's a recurring idea in this poem. Dickinson's choices seem to deliberately invest the snake, or the whip in this case, with human abilities and powers. The line between the inanimate and animate (human) world seems to be purposefully blurred here.
See how much Dickinson can pack into these seemingly-simple little lines? Color us impressed!