It sifts from Leaden Sieves -
The “it” is snow, for which the rest of the poem invents a series of metaphors. A sieve is this:
It’s used to sift flour to make it fluffier. Here, “Leaden Sieves” is a metaphor for lead-colored (gray) winter clouds.
To help improve the meaning of these lyrics, visit “It sifts from Leaden Sieves - (311)” by Emily Dickinson and leave a comment on the lyrics box