The poem
This darksome burn, horseback brown, His rollrock highroad roaring down, In coop and in comb the fleece of his foam Flutes and low to the lake falls home. A windpuff-bonnet of fáwn-fróth Turns and twindles over the broth Of a pool so pitchblack, féll-frówning, It rounds and rounds Despair to drowning.
Degged with dew, dappled with dew Are the groins of the braes that the brook treads through, Wiry heathpacks, flitches of fern, And the beadbonny ash that sits over the burn.
What would the world be, once bereft Of wet and of wildness? Let them be left, O let them be left, wildness and wet; Long live the weeds and the wilderness yet.
It took me awhile to understand the first stanza, but once you understand the theme(water) it is not so hard.
The first line, I think, is talking about dark water puddles. "Rollrock" is talking about the round stones in a creek because they have been subject to erosion. So, "rollrock highroad" is where a stream flows. The stream is flowing and foaming, and arrives home at the lake.
Also, Hopkins was a Scottish man, and I assume he is talking about the Scottish landscape . . .