There's a certain Slant of light, Winter Afternoons— (1-2)
From the very beginning we sense the speaker's amazement through her use of ambiguity in words like "certain" and "Slant of light." We're not meant to see specifics because she herself can't fully grasp the awesomeness of that mysterious light.
None may teach it—Any— (9)
It's impossible to teach this slant of light and its mysterious effects that adds to the speaker's sense of amazement. Those dashes also serve to create a sense of dissonance, furthering the impossibility of fully capturing or teaching it.
When it comes, the Landscape listens—Shadows—hold their breath— (13-14)
Even the landscape can't help but be amazed by the light and its mysterious ways. The personification of the landscape and shadows also serves to reflect the speaker's sense of awe. Everything is listening and standing still.