This link will take you to Shmoop which has lots of useful information. For our study you should read about the poem and then focus on the theme of man and nature.
the natural sublime: Majestic natural landscapes that, when contemplated, lead to enlightenment (often healing or higher spirituality). The landscape is often represented as majestic and all-powerful, giving insight into the human condition - mortality, insignificance, imperfection, futility, etc.
hubristic: excessively proud or over-confident. Hubris is the pride that comes before the fall.
Significance
The significance of Coleridge's Kubla Khan is its Romantic representation of landscape. The Romantics evoke the natural sublime in their literature, depicting natural landscapes as a majestic representation of perfection and a reminder of God's supreme power. In Kubla Khan, the landscape of the river and its caverns surpasses even the most amazing of man-made places - Xanadu. Compare the almost off-hand descriptions of Xanadu with the wildness of the river.
We could also see Xanadu as a metaphor for man's illusion (delusion) of control over nature. On the surface, Coleridge depicts this protected oasis - a representation of man's supreme dominance. But underneath runs a river capable of both peaceful calm and terrible power.
So Romantics represent natural landscapes as all-powerful - a reminder to man of his ephemeral nature. We are reminded that our best efforts to surpass God's majesty are hubristic and feeble by comparison.
Romantic poets also believed that with music, words and images, they could give immortality to ideas, places and experiences. The final stanza of the poem may well represent Coleridge's inability to fully achieve this end as he has forgotten the rest of it.