There's a certain Slant of light, Winter Afternoons—
The speaker brings us into the poem's setting immediately. We get that it's winter, afternoon, and the air has a certain "Slant of light" that's typical during this time of day and year.
We don't know exactly what the speaker means by "Slant of light," but seeing how "Slant" is capitalized (and the first line is the poem's title), we understand that it's going to be important.
When we imagine winter afternoons, we can kind of see what the speaker is talking about. The sky is often overcast and there's usually some sort of snow or frost on the ground. So the light tends to reflect off of all these surfaces, making things look bright in an odd way.
Notice the speaker's diction that's not super-formal or ornate and sounds pretty casual. We have words like "there's" and "certain" that indicate the speaker's informal kind of voice. She's not looking to be overly descriptive or flowery in her words. (Check out our "Speaker" section for more on her.)
We also have more capitalization of common nouns and adjectives in line 2: "Winter Afternoons." Dickinson does this sort of thing a lot, usually in an effort to point out the significance of specific words or phrases. The dashes are also pretty common in her poems and act kind of like periods that simultaneously serve to bridge ideas together. Maybe they're a blend of a period and a comma. So let's see how they work in the rest of the poem…
That oppresses, like the Heft Of Cathedral Tunes—
By line 3 we're seeing a bit more of this "Slant of light" that in this line "oppresses." So we get the feeling that this kind of light is powerful, but not in a happy sunshine kind of way.
The speaker uses a simile to compare the light to the "Heft" (significant weight) of "Cathedral Tunes." So with the figurative language we see here, we can assume she's thinking of pipe organs and church "tunes" of that nature.
So this slant of light is oppressive in the same way the speaker may feel oppressed by the weightiness of cathedral music. Notice that we have more capitalization of key words like "Heft" and "Cathedral Tunes," so we know these words are significant to the speaker's ideas.
By line 3, we're also beginning to understand the angle the speaker is choosing to use to describe this slant of light. We're meant to see this kind of "light" in a different sort of way that's not immediately uplifting in the way we normally see light described.
Here, with all the capitalization and use of words like "oppresses," we see light in a more painful and isolating way.
And again, when we imagine winter afternoons, we understand the imagery and its associations even more. The overcast light we get, the low angle of the sun, and all the surfaces this light can reflect off of, make this kind of light powerful, but not in a happy sort of way. It's more like the kind of light that hurts your eyes and is therefore "oppressive."
And for the speaker, perhaps there's a connection between this kind of oppressive light and the religious connotations of "Cathedral Tunes." The tunes often sound quite powerful but in a sense perhaps they're equally oppressive to her because of their powerful sound.
By the end of the first stanza, we can also spot the sort of "common measure" Dickinson is working with. The rhyme scheme follows the ABCB pattern with the second and fourth lines being exact rhymes ("afternoon" and "tunes"). The meter seems to roughly alternate between iambic tetrameter and iambic trimeter for each line. That's just fancy talk for saying we have a stressed/unstressed pattern that equals four stressed syllables in one line (iambic tetrameter) and then the same pattern occurs in the next line with only three stressed syllables (iambic trimeter). Check out "Form and Meter" for more on that good stuff.