The speaker is describing, as earth's own poetry, the sounds made by living creatures, and the visual poetry created by the appearance of flowers in bloom and birds flying. He suggests that these sounds and visuals are all expressions of earth. If they are earth's poetry, then, the poetry of earth is never dead because there is always something living and alert when other things have been sent into hiding by "the hot sun" or the time of year.

Specifically, in this poem, Keats refers to the grasshopper, stating that his "voice" can be heard when the summer is too hot and the birds have gone to hide. In a similar way, when it is cold, and frost has "wrought a silence" because all other creatures are too cold to emerge and the flowers have died, there will be another song audible—that of the cricket. Essentially, the poetry of earth can never die because animals emerge at different times of year to pick it up and express it, particularly the cricket and grasshopper, who take over from each other as summer becomes winter.

Keats means that, whatever time of year it is, there's an underlying life force in nature that never dies. Whether it's in the extreme heat of summer or the freezing cold of winter, nature is always alive, always beautiful. When the poem begins, it's summertime, and the weather's so hot that the birds stop singing, seeking relief from the burning sun in the cool shade of the trees. Yet nature still lives on in the figure of the grasshopper as he hops about, chirping merrily away.

In winter-time nature's life force manifests itself in the cricket. While all is silent on a cold winter's night, the cricket loudly sings his song, basking in the warm glow of a stove. What Keats is emphasizing here is the continuity of nature. Though completely different on the surface, the seasons are linked together, part of the same unified whole.