There has been lots of critical discussion about the precise "meaning" of this amazing poem. Some have suggests that this poem is all about the act of creation and imagination, as expressed in the visions that the poet sees. In lots of ways, however, the poem defies interpretation or categorisation being imposed on it, so there is no definite answer to your question!
Personally, I do think this poem has a lot to say about the act of creation. We start off with a description of Xanadu, which is clearly an act of creation in itself, representing a huge triumph of imagination in its construction:
So twice five miles of fertile ground
With walls and towers were gridles round:
And there were gardens bright with sinuous rills,
Where blossomed many an incense-bearing tree;
And here were forests ancient as the hills,
Enfolding sunny spots of greenery.
The author is clearly inspired by this tremendous act of creation, which is described as "a miracle of rare device," and we see in the last stanza the desire of the speaker of the poem to copy Kubla's act of creation through music, inspired by the Abyssinian maid as a muse. The narrator likewise wishes to impose order through his act of creation in a dangerous universe, and be thought of by society as an artist, one whose power to create and force of the imagination makes him feared:
That with music loud and long,
I would build that dome in air,
That sunny dome! those caves of ice!
And all who heard should see them there,
And all should cry, Beware! Beware!
His flashing eyes, his floating hair!
Therefore, arguably, this poem has much to say about the act of artistic creation, the triumph of the imagination, and the way that society treats artists: those who are able to create their works of art out of thin air.
I memorized Coleridge’s poem “Kublai Khan” for a 9th grade English course; some 25 years later, I can still recite it, even if I occasionally miss a line or two. I still don’t fully understand how all the parts of the poem fit together, though. It may be a challenge to find a single theme that captures the entire poem. Two possibilities might be Imagination and Creativity or Opulence.
Imagination and Creativity: The poem is often read in light of Coleridge’s opium addiction or his view, as a Romantic, that poetry is the product of the inspired moment.
Opulence: The rich imagery is, simply put, what makes this poem so successful. From “caverns measureless to man” in the opening lines to the “flashing eyes” and “floating hair” at the end, the poem is intensely visual.
The links below lead to some very good discussions of the poem and its possible themes.