A "pastoral" is a poem that idealizes rural life and landscapes. By referring to the Grecian urn as a pastoral, Keats equates it to a literary work. Indeed, he has spent the previous stanzas recounting the tales that the urn tells and even some it doesn't tell (when he imagines the vacant "little town" that the characters on the vase have left behind). The urn's etchings contain characters, such as the youth and the maiden he pursues, and actions that suggest some kind of plot to Keats. If poetry is "emotion recollected in tranquility," as Wordsworth suggested, then the urn could potentially qualify. Keats, at any rate, draws much emotion from the drawings, including "more happy, happy love!" In the first stanza, Keats compares the urn to a poem when he calls it a "sylvan historian, who canst thus express a flowery tale more sweetly than our rhyme." In the final stanza, he returns to that metaphor by addressing the urn as "Cold Pastoral."
It is easy to see, then, that Keats's metaphor of the urn as a poem idealizing country life is valid. Why would he call it "cold" when he finds it expressive of such warm emotions? It is cold in two ways. Glazed ceramic pottery has a cold sensation to the touch. In addition, Keats means that it is not alive. The scenes it captures are of life: active people, growing plants, lowing animals. But since it isn't alive, it can never die; therefore, its message will live on to inspire and delight people in ages to come. Although "cold" may seem to have a negative connotation, Keats shows how its coldness is a benefit because it makes the urn almost eternal.
By calling the urn a "cold pastoral," Keats means that it is a form of poetry that will endure for ages to come, continuing to spur the emotions and imaginations of many others just as it has delighted him.