"On the Grasshopper and Cricket" begins with a fervent declaration: "The poetry of earth is never dead." This idea might feel equal parts moving and mysterious at first. What, readers might ask, is the "poetry of earth"?
In the first quatrain, the answer (or at least part of it) is "a voice"—a constant voice, one that travels across the summer fields even when it's so blazingly hot that the birds are "faint" and fall silent in the shelter of the "cooling trees."
The imagery in these first lines makes this scene seem both comforting and oppressive. The "voice" the speaker describes travels "from hedge to hedge about the new-mown mead." In other words, it runs through the hedgerows that separate freshly shorn fields. This is a comfortable English countryside scene, a domestic spot of "earth," not some grand mountainous vista.
If even the birds are "faint with the hot sun," though, this is a comfortable scene on a seriously uncomfortable day. The image of the birds withdrawing into the trees suggests that it's so hot out that the world has fallen almost silent: all the animals can do is rest and pant. (While the speaker isn't present in the scene, readers might imagine that the human population is sweating and suffering, too.)
All through this discomfort, though, the "voice will run." Notice the way the speaker confidently uses the future tense: the voice isn't just running now, it "will" run always. The sun can't burn it away; it's a constant thread that "run[s]" through even unpleasant times.
This sentiment might feel familiar to readers who've encountered the famous first words of Keats's later book-length poem Endymion: "A thing of beauty is a joy for ever." The "voice" that speaks of the "poetry of earth," too, is a thing of beauty that doesn't stop giving joy even when circumstances are tough.
The first quatrain of this sonnet is grounded in a familiar (and not altogether idyllic) landscape. But it's also full of a very Romantic faith in the enduring, consoling beauty of nature and poetry. Eternal loveliness runs right through the (sometimes uncomfortable) day-to-day world. In the second quatrain, Keats will hug that combination of the ordinary and the enchanted even closer when—perhaps unexpectedly—he identifies the source of the mysterious voice.