Thou still unravish'd bride of quietness,
Though foster-child of Silence and slow Time,
Sylvan historian, who canst thus express
A flowery tale more sweetly than our rhyme:
What leaf-fringed legend haunts about thy shape
Of deities or mortals, or of both,
In Tempe or the dales of Arcady?
What men or gods are these? What maidens loth?
What mad pursuit? What struggle to escape?
What pipes and timbrels? What wild ecstasy?

Heard melodies are sweet, but those unheard
Are sweeter; therefore, ye soft pipes, play on;
Not to the sensual ear, but, more endar'd,
Pipe to the spirit ditties of no tone:
Fair youth, beneath the trees, thou canst not leave
Thy song, nor ever can those trees be bare;
Bold Lover, never, never canst though kiss,
Though winning near the goal—yet, do not grieve;
She cannot fade, though thou hast not thy bliss,
For ever wilt though love, and she be fair!

[…]

O Attic shape! fair attitude! With brede
Of marble men and maidens overwrought,
With forest branches and the trodden weed;
Thou, silent form! Dost tease us out of thought
As doth eternity: Cold Pastoral!
When old age shall this generation waste,
Though shalt remain, in midst of other woe
Than ours, a friend to man, to whom thou say'st,
'Beauty is truth, truth beauty,--that is all
Ye know on earth, and all ye need to know.'

This the beginning of Keats' famous ode—he's waxing lyrical about the scenes painted on the side of an old Greek urn.

There are figures and scenes etched into the side of this Greek urn, and the speaker's looking at them. Simple enough, right? The urn is really mysterious to this speaker, but it excites his imagination.

Why is the urn worthy of a poem? Because the scenes on it seem to defy time: the "[f]air youth" beneath the trees is always going to be piping away on his flute, the trees under which he sits will always be green. The beautiful woman who the lover is chasing on the side of the urn will never grow old. This fragment from the ancient past seems to freeze time, to stop it from moving forward.

An ode is a poem that's written in praise of someone, or something, and it's usually written in high poetic style, often with fancy vocabulary and complicated stanza structure. We can see all of these elements in Keats' poem.