This poem is not very easy to summarize because it is not really about anything -- it doesn't have a plot.
The great majority of the poem is just a description of the "pleasure dome" that Kubla Khan has supposedly constructed. Essentially all of it is like this until the line "A sunny pleasure-dome with caves of ice!"
After that, Coleridge brings in the Abyssinian maid playing the dulcimer. Her song is supposed to remind him so much of Kubla Khan's pleasure dome that he could "build it in the air."
So the poem is really more about the imagery used than it is about any storyline or plot.
Critics argue about the meaning a lot, but the most common explanation for the poem is that it is about the nature of poetic inspiration. It is about how hard it is for poets to capture in words the visions they see in their heads. Coleridge, in the poem, could build this whole fantastic place again, if only
Could I revive within me
Her symphony and song