The poem starts with the speaker noting a "certain Slant of light" that falls during "Winter Afternoons." "Slant" refers to an angle; think of the way beams of light might angle through a window or trees on a cold afternoon. The speaker will go on to talk about the poignant emotions this light evokes, but even now there's a sense of something being off; light is often associated with life, truth, hope, and warmth, but this light is slanted, at an angle. It's also notably coming through in the winter—a time typically associated with hopelessness, barrenness, and death (the afternoon is also associated with the latter half of a day). As such, there's immediately a sense of tension here. This paradoxical image anticipates the "internal difference," or confusion, that this beam of light prompts in the poem's later stanzas.
And, as is typical with Dickinson's poetry, even this apparently simple image introduces ambiguities: who is the speaker? Are they seeing this beam themselves, or are they generalizing about humanity? The first question is never settled; to the second, one can answer "both," since "There's" could mean both "there it is in front of me" and "there exists out there." The capitalization of "Winter Afternoons" treats the phrase like a proper noun, further suggesting that the speaker is talking about a general experience shared across humanity.
The meter of these two lines reflect the poem's tentative tone:
There's a certain Slant of Light,
Winter Afternoons ‒
The meter is highly irregular. The most common foot is the trochee ("certain," "Slant of," "Winter" ), which gives a jerky, stop-start rhythm to these lines, mimicking the speaker's halting thought process.
Line 2 then ends with Dickinson's characteristic dash, which she uses instead of a variety of more typical punctuation, including semi-colons and full-stops. This dash gives a sense of incompleteness to the line, akin to the speaker's urge to drill deeper and deeper into each thought and emotion being experienced. These dashes also lead the reader down and on through the poem as well.
The unusual phrase "Slant of light" recalls another Dickinson poem, “Tell all the truth but let it slant” (1263). In this poem Dickinson writes, “Success in Circuit lies.” The association between light and truth is common, but Dickinson’s angle, that to get at truth requires going in a “Circuit”—that is, approaching it from an angle rather than head on—is her own. This idea ties in with the way that the beam of light in this poem acts as a messenger from heaven, which cannot deliver its truths directly, but requires nature as an intermediary (in a way requiring a circuitous path to reach humanity).