The poem speaks about the Emperor Kublai Khan, who had been known to the Western public through the travels and works of Marco Polo. Khan was said to be extraordinarily powerful, both militarily and in influence; for him to build a giant underground palace was in-tune with the common Western views of China. The well-known opening stanza of "Kubla Khan" is as follows:
In Xanadu did Kubla Khan
A stately pleasure-dome decree:
Where Alph, the sacred river, ran
Through caverns measureless to man
Down to a sunless sea.
So twice five miles of fertile ground
With walls and towers were girdled 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.
(Colridge, "Kubla Khan," eNotes eText)
These lines describe some of the amazing features of the "pleasure-dome" in Xanadu. The incredible location -- underground, near a river in the caverns -- is mentioned, as are the various features, all of which seem to flourish without direct sunlight. It is described later as a "sunny" pleasure-dome, so perhaps there were shafts in the rock ceiling. Regardless, the main theme is the opulence and extraordinary imagery of the work; Khan orders the dome built for his pleasure, and places it in an extraordinary place because it pleases him to do so, and to have something unique. The poem also sets up some of the darker imagery, with the "sunless sea" being separated from the lush gardens and forests that surround Xanadu. These themes inform the rest of the poem, which shifts from the concrete to the spiritually abstract.