Keats is writing about a Grecian urn which has illustrations all around it. He is trying to imagine himself inside the world represented by the urn and is pretending that the illustrations are reality. These are "poetic conceits." For example, there is a picture of a priest leading a heifer to an alter to perform a sacrifice to the gods. Because this is festival, the "little town" is emptied to attend the ritual. Keats pretends that since the scene is frozen in time the inhabitants will never return to the town, and the town itself will never know why it is left desolate. Everything is frozen in time. The urn is perhaps almost two thousand years old. The pipes have been playing for all that time and will go on playing (to the imagination) for another two thousand years or more. The bold lover who is endeavoring to kiss the maiden will never quite achieve his goal, but she cannot grow old and he will be able to go on loving her forever. These are all fancies of the poet aroused by looking at the pictures beautifully preserved in the ceramic art and encircling the entire urn. Keats is discussing each in turn as it appears on the urn. It is pure imagination, just as it is pure imagination in his "Ode to a Nightingale" in which he enters the nightingale's world and travels back in time to biblical days and then travels on to a fairyland with magic casements opening on the foam. The descriptions will appear to be in order and to be logical if you imagine the poet examining them as they appear on an ancient Grecian urn.