In "Ode on a Grecian Urn," the speaker remarks upon the intriguing beauty of the scenes he sees depicted on the surface of the eponymous urn. In one such depiction, the speaker sees a "happy melodist," a piper, and states,
Heard melodies are sweet, both those unheard
The implication here is that the music suggested by the picture is sweeter and more beautiful than any music that the speaker might really hear. This is perhaps because the picture, in combination with the speaker's own imagination, evokes an ideal melody to which no real music can possibly compare.
In the third stanza of the poem, the speaker describes the trees he sees on the side of the urn. He implies that these trees are more splendid and more beautiful than any trees that might exist in the real world, because the trees on the urn "cannot shed / [their] leaves, nor ever bid the Spring adieu." In other words, the trees on the urn are more beautiful than any real trees because the trees on the urn will remain beautiful forever. Their beauty will not alter or fade with the seasons, as the real trees will.
For these reasons the speaker determines that art is profound and beautiful and, in some ways, more beautiful even than the life which art seeks to imitate. Gazing upon the urn teaches the speaker that the true value of art lies firstly in its ability to appeal to the infinite possibilities of the human imagination and, secondly, in its ability to immortalize beauty. This realization is the most significant effect that the urn has upon the speaker.