In Keats's period, artifacts from Ancient Greece were flooding into England, and Greco-Roman art was much admired. Keats had seen many drawings of Grecian urns and made a copy of a particular work, the Sosibios vase, although the urn of this poem is not based on any one specific piece. He was, overall, responding in this poem to the general admiration for ancient Greek culture.
"Beauty is truth, truth beauty" comes very near the end of the poem, in the line before the last. Keats imagines the urn saying it to humankind. The statement is a tautology, for to say beauty is truth is the same as saying truth [is] beauty. Keats never, however, defines what either word means. Critics have debated whether the line adds to the poem or detracts from it: I like the idea of the urn "speaking" to us, but at the same, I see the line as Keats being out of ideas and needing to find a way to wrap up the poem. It's almost as if he is saying, "I don't know what this means: just enjoy its beauty."
This leads to Keats's use of numbered sections. In the first two sections, the poet examines the picture on the urn with great curiosity, and as he describes it, he enters into the scene before him more and more wholly with his imagination. In the third stanza, right in the middle of the poem, Keats's emotions hit a crescendo as he contemplates being a figure on the urn. To be one of those people would mean to be young forever, always happy, always in love, always on one's way to a festival in spring. Wouldn't that be fantastic?, he thinks. We know he is ecstatic in this stanza because he repeats the word "happy" over and over and over again and uses exclamation points.
In the fourth and fifth stanzas, Keats comes down from his high and moves away from his ecstatic identification with the figures on the urn. He thinks about the emptied town that the joyous people come from and then contemplates what the urn means as he gains some distance from it and regains control of his emotions.