Our poet/narrator is contemplating a Grecian urn, of course, and he sees and makes observations about life. He sees two scenes"
First, he sees a young man wooing a beautiful young woman under a tree; they are about to kiss. The contemplation is
Bold Lover, never, never canst thou kiss,
Though winning near the goal yet, do not grieve;
She cannot fade, though thou hast not thy bliss,
For ever wilt thou love, and she be fair!
You may not get to kiss her, he says, but she will always be beautiful. There is also a tree in full bloom whichwill never have to suffer the loss of its leaves, and the young man playing his pipe will always be young anf have his passion--and his song will never grow old.
Then he sees a scene of a quaint, empty town.
What little town by river or sea shore,
Or mountain-built with peaceful citadel,
Is emptied of this folk, this pious morn?
And, little town, thy streets for evermore
Will silent be; and not a soul to tell
Why thou art desolate, can e'er return.
What town it is and why it's empty will forever be a mystery.
The first scene, while beautiful and everlasting, is sad to him because the promised action of the scene will never happen: no kiss, no love, no spring, no music.
In the second scene, the town will always be beautiful but it will also forever be empty, not serving the purpose of a town which is to hold a teeming, living community of people.
The narrator is reflecting on the fact that the history (story) of the urn will last far longer than anything he might write or any story one might live, yet it is not alive. Life, while not always so beautiful and perfect, does have love and birth/death and music and passion--even if it's ugly sometimes.
His final lines are up for debate and speculation, though they are the most famous and oft-quoted:
"Beauty is truth, truth beauty,--that is all
Ye know on earth, and all ye need to know."
Keats himself saw the ambiguity of these lines as a flaw, but that's what poetry is supposed to do--allow for personal reflection.