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.