Stanza 1, the speaker is addressing the urn itself. He mentions it is timeless and has lasted for a long time and asks questions of the message on its side--who are these lovers? These gods? The story behind the chase and the piper? Stanza 2, he says he can imagine the tune the piper plays and it is sweeter than one actually heard simply because everyone who imagines the tune will imagine one that would sound sweet to him...therefore the imagined tune is sweeter to everyone. He also mentions the trees whose leaves will never fall and the lover always on the verge of that first kiss. The girl will always be lovely, it will always be Spring. How wonderful to be stuck in that position. Stanza 3, he continues with the idea of being stuck in the good part of love and weather. The lovers will never experience vengence or hurt, and the spring weather wil always be balmy, not hot and parching. Stanza 4, he speaks of the cow decorated with flowers being led by a priest to the sacrifice. The town is deserted and no one will ever know why since it's not on the urn...the water is ever present and the streets are forever silent. The people can never return home since they were not painted onto the urn's story. Stanza 5, he mentions the shape of the urn and the stories etched there forever. He bemoans that after we are dead the urn will still exist. Cold, hard beauty and truth remains. It is all we need to know.
Keats was inspired to write this poem after seeing the scenes painted on an ancient Greek urn. The eNotes introduction points out that the speaker "attempts to identify with the characters because to him they represent the timeless perfection only art can capture."
There is not enough room here to give you the meaning of every line in the poem. If you could point out a few lines that are difficult for you, that would be more doable.
Take a look at the eNotes summary and discusion of themes in the poem; I've pasted links to them below. I've also given you a link to an article on how to explicate, or analyze, poetry. You should find a lot of useful informaton there.