Alliteration is a trademark device of Gerard Manley Hopkins, and here appears in almost every line. Altogether, the many varied instances of alliteration add to the beauty of the poem's language—reflecting, on a linguistic level, the "pied beauty" that the poem celebrates thematically.
Even the first word of the first line is alliterative. The hard /g/ sound that links "glory" to "God" is a common association—not one that has been invented here. It speaks to the belief that God made the world, that the world is full of God's majesty, and that God is therefore deserving of praise.
In line 2, alliteration combines with consonance (of /l/ sounds) and assonance (of short /u/ sounds) to create the fresh phrase, "couple-colour." This, essentially, just means "two-colored." But the deliberate deployment of similar sounds means that the word itself feels like it has two colors, one which is based on the alliterative /c/ and the other which is constructed around the /l/ sounds of the second syllables of each word. The phrase also chimes alliteratively with "cow" at the end of the line, which not coincidentally is the image that the poem is using to describe the kind of skies it's talking about.
Later, in line 4, four words out of six begin with an /f/ sound: "fresh," "firecoal," "falls," and "finches." This very subtly evokes the noises people make when handling or blowing on something hot, and also creates a sense of abundant beauty (which, indeed, is one of the poem's main aims).
This /f/ sound continues into the second half of line 5, which also introduces an alliterative /p/ with "plotted," "pieced," and "plough." This /p/ sound is further linked via consonance to "stipple" in line 3, which is a word that describes a visual dot effect; the many instances of this percussive /p/ sound is perhaps an auditory reflection of the visual nature of "stippling." The /p/ and /f/ sounds in line 5 also represent the way in which humans work the land through agriculture, the line itself sounding as if these sounds have been planted there to grow.
Lines 8 and 10 continue with this /f/ sound, with line 10 providing the key phrase "fathers-forth." The /f/ is linked to the idea of a paternalistic God, making all the other instances of /f/ seem like evidence of God's design for the world.