In "The Grasshopper and the Cricket" Keats uses nature imagery to paint a two-sided picture of life on earth. The first is a direct description of how human life observes and intersects with nature on earth (of course Keats never speaks of "nature," he only speaks of the things belonging to nature: birds, trees, crickets, grass, etc.).
The first and direct story told by the poem is that in summer, when the birds faint from the heat of the sun, they take shelter under the cool leaves of trees and listen to the grasshopper as he sings and plays about the meadow. When the grasshopper is finished playing , he reposes himself in the cool of a weed. This is observed by humans.
Then in winter, nature intersects with humans. While the world is frosted in cold, a cricket sings from the warm stove. The human listener, in half a doze, is reminded of the luxury of summer verdure and imagines he hears the sound of the grasshopper.
For Keats, as a poet of romanticism, nature communicated through symbolic emblems, which poets copied by making symbolism highly important in their poetry. The second story is a metaphorical one. The hot sun and summer luxury symbolize youth and health. The frosty cold winter during which the imagination (for the Romantics, imagination was the highest order of thought that governed cognitive reason) recalls the youth and health of summer.
In this context, nature as a whole is emblematic of the individual joys and pleasures of a life lived through all the seasons of all the years and through the metaphoric seasons of lifetime.
Bear in mind that in 1816 (the year Keats received his apothecary license) when Keats wrote this poem, he had already lost his father and mother, who died of tuberculosis, and was nursing his brother Tom who was also dying of tuberculosis. This is when Keats contracted the disease that also led to his untimely death in 1821.