The speaker imagines a cloak that holds the whole outdoors: winds, clouds, trees, and a green lining where rain runs down in "satin streamers." It is a strange, lovely picture, part clothing and part landscape. The speaker holds the cloak open, filled with air and decorated with a "mimic earth," then fastens it shut and lets it drop.
That drop is where the poem changes. The short lines, "Let all / Collapsing over / Cover me," slow everything down and mimic the cloak falling. From outside, all that remains is "a still dark heap," plain and lifeless. But the last line claims something bigger: "inside I am now my cloak." The speaker has stopped being a person wearing nature and has become the world inside the fabric.
The mood is quiet, private and a little magical. It feels like pulling a blanket over your head and finding a whole world in the dark, safe and slightly hidden from view.