The poem is built around a small trick of perception. For two lines we seem to be looking at snow: a whole hillside turning white, and "prematurely" suggests winter has arrived before its time. Then the last line, "fall webworms," changes everything. The white is not snow at all but the silky nests of caterpillars, spread across the trees in autumn.
The word "fall" does double duty. It names the season and also the insect, so the poem quietly keeps both ideas in play. The speaker doesn't say how they feel, but the tone seems to be one of surprised noticing. There's a hint of unease in "prematurely," as if something is slightly off in the year's timing, yet the ending is matter-of-fact and almost gentle. The mood is quiet and observant. Nature offers a false winter, and the poem lets us feel the small jolt of being fooled before it shows us what is actually there.