There are several pictures painted on the urn's sides. Read each stanza to get more out of this poem and to visualize the pictures described: The first stanza addresses the urn asking questions of the stories it holds in its pictures: It is a historian, leaf-fringed (the border along the urn, probably) that tells a story of gods, mortals, maidens, etc.
Heard melodies are sweet, but those unheard Are sweeter; therefore, ye soft pipes, play on;
Some one playing a horn of some type--we can't hear it but imagine the sound, so it is sweet to our ears as we imagine music we would enjoy.
Fair youth, beneath the trees, thou canst not leave Thy song, nor ever can those trees be bare; Bold Lover, never, never canst thou kiss
There is also a young couple under some trees just about to kiss, but will always be trapped in that exciting moment--never to kiss, but never to grow old, and always in love and just on the verge of bliss.
Ah, happy, happy boughs! that cannot shed Your leaves, nor ever bid the Spring adieu; And, happy melodist, unwearied, For ever piping songs for ever new;
The trees are happy since they will always be in their prime, in Spring, full of leaves and there is a young person (happy melodist, probably male) who will happily play forever.
Who are these coming to the sacrifice? To what green altar, O mysterious priest, Lead'st thou that heifer lowing at the skies, And all her silken flanks with garlands dressed? What little town by river or sea shore, Or mountain-built with peaceful citadel, Is emptied of this folk, this pious morn?
Here is a picture of a priest leading the cow to sacrifice, and she is all decorated with flower garlands. The town is empty, and by the water and there are mountains in the background. He questions where the people are--at a celebration, perhaps, never to be seen or to return since they are not pictured here.
The urn is again addressed as a "cold pastoral"--something which can not answer his questions yet displays the depictions of daily life on its side in a quiet, silent, historical manner. It has survived all this time to stand quietly in beauty.