What important Romantic concept is represented when the author describes nature as "the burthen of the mystery, / in which the heavy and the weary weight / of all this unintelligible world, / is lightened..."?

- Transcendence.

Explanation:

The romantic poets were deeply associated with portraying the subjects that set the humans apart from the limits of this materialistic world and take them to a whole imaginative spiritual world where they can purify their soul and liberate themselves. For example; Coleridge's ideas of a spiritual place named 'Xanadu' in 'Kubla Khan'.

As per the question, 'transcendence' is the most significant romantic aspect that is reflected through the given quote about the description of nature as the description involves the 'state of being free from the constraints or shackles of this material world('the burthen of the mystery...heavy and the weary weight of unintelligible world').