This poem celebrates the scenes and stories painted on the Grecian urn. The speaker addresses the urn with several rhetorical questions and then proceeds to draw the conclusion that the subjects on the urn are sweeter and happier than the humans on earth.
The speaker first notes that
Heard melodies are sweet, but those unheard
Are sweeter; (11-12)
meaning that the mortal tunes on earth will fade, but the concept of musical beauty suggested by the urn will not.
He goes on the address the picture of the lover, just bending forward for a kiss from his lady. The speaker acknowledges that he will never, ever get his kiss, but that this moment of expectancy and thrill is far more powerful than the kiss itself. He tells the young man:
Bold Lover, never, never canst thou kiss,
Though winning near the goal - yet do not grieve;
She cannot fade, though thou has not thy bliss,
For ever wilt thou love, and she be fair (17-20)
Even though they will never become full lovers, the will also never age, fight, argue, cheat etc. In this way, love on the urn is much more permanent than love between mortal men.
The speaker goes on to assert that the people on the urn live in perpetual spring and are perpetually young; this does not happen in the "real world". Finally, he notes in the final stanza that when we are all dead and gone, the urn will remain as truth:
Beauty is truth, truth beauty - That is all
Ye know on earth and all ye need to know (49-50)