It is always difficult to guess what a writer actually meant to convey, especially if they are dead and we cannot ask them! However, what we can assess is what message a poem actually does convey, whether that was what the poet intended or not; hopefully, however, if the writer is skillful (as Keats was), we can imagine that they actually conveyed whatever message they intended.
In this poem, Keats conveys the beauty of even the smallest creatures, like the grasshopper or the cricket—two of nature's creations that human beings might be inclined to overlook because they are so small or seem so inconsequential. He draws our attention to them here. At the same time, Keats seems to be expressing wonder and awe at all of nature and "The Poetry of the earth" (line 1). Even when the birds tire, the grasshopper still sings his own song in the summer. Then, in winter, when the frost has made everything else seem silent, the cricket still sings. Though we do not always notice it, nature has a poetry that never stops, and Keats tries to draw our attention to its beauty.