Anytime you hear a religious poet, like Hopkins, using the word "fall," you have to assume that he or she is thinking not only of the season, but also of the "fall of man"—a.k.a. Adam and Eve eating the forbidden fruit in the Garden of Eden and getting kicked out of paradise in the Judeo-Christian creation story. But as a season, "fall" also has symbolic connotations in poetry: if you think of the seasons as symbols for a person's life, with springtime being childhood, summer being youth, and winter being old age, then "fall" would be middle age, but signaling a turning point towards getting nearer to death. There are lots of possible symbols here. Check it out!
Title: Hopkins lets us know that "fall" will be important right from the title. And by juxtaposing, or contrasting, "fall" with "spring," he lets us know that the poem is going to be about contrasts of youth and old age, innocence and experience, life and death.
Dedication: The poem is dedicated "to a young child." Because the title of the poem is "Spring and Fall," the dedication tips us off that the "young child" must represent the "spring" half of the contrast set up by the title. Does that mean that the speaker represents "fall" because he's older and more experienced? Or is "fall" just the setting, since the poem takes place during the autumn? Or could it be both?
Line 2: "Unleaving" is a word that Hopkins made up to describe the way that leaves fall off of trees in the autumn. The speaker and the child he addresses are in a forest called "Goldengrove," which sounds like a magical, fairytale kind of place—partly because of the alliteration, or repeated G sound, and the assonance, or repeated long O sound, of the name.
Line 8: Two made-up words (also known as neologisms) can be found in this line. "Wanwood" means dead leaves, but the word "wan" means pale and sickly, so the made-up word "wanwood" suggests disease and sickliness. The repeated "w" sound of "worlds of wanwood" is another example of alliteration. "Leafmeal" is another made-up word—it describes leaves lying in a haphazard, disorganized way. It's like the word "piecemeal," only it describes leaves. And we've got still more alliteration with the repeated "l" of "leafmeal lie." Hopkins loved his alliteration! All of these made-up words and alliteration have a couple of possible effects—it makes the poem sound more song-like, and it might also be intended to make it sound more like a poem addressed to a child, since kids are often good at making up words.
Lines 14-15: At the end of the poem, the speaker comes to the conclusion that Margaret isn't just boo-hooing over the fallen leaves, she's grieving over the "blight" that all people have in common—the knowledge that everyone will die one day. Could it be that the "fall" referenced in the title describes Margaret's metaphorical "fall" from innocence into the knowledge of death and mortality?