Keat's poem is an Italian or Petrarchan sonnet, which consists of an octet (group of eight lines) followed by a six-line sestet. The last two lines of this sonnet, part of the sestet, bring together the two sections of the sonnet to end on a theme dear to the heart of Romantic poets: memory.
In the first part of the sonnet, Keats's speaker describes the sounds of nature in summertime. He says these sounds, which he calls the "poetry of the earth," are "never dead." To support this idea, he states that when the birds get too tired to sing, the grasshoppers takes over.
In the second section of the sonnet, Keats moves to winter. Instead of saying that the poetry of the earth is never dead, his speaker notes here that it never "ceases." Here, the speaker sits inside by a warm stove, away from the "frost" outside. Indoors, encouraged by the stove's warmth, a cricket "shrills."
The last two lines bring summer and winter together, because as the winter cricket shrills, this sound merges in the drowsy speaker's mind with the remembered song of the grasshopper on the grassy hill. The sound of the cricket triggers a happy memory of summer's natural beauties. We now understand the subtle shift from nature's poetry "never [dying]" to "never [ceasing]": the grasshopper might be dead, but his song lives on in the speaker's memory and, blending with the sound of the cricket, brings comfort on a cold winter night.