Shelley starts in the middle of a grievance. The passage opens on "Where", a word that promises a place, and then spends four lines on what has been done to it. The grammar holds back: "Where man's profane and tainting hand / Nature's primaeval loveliness has marred". We wait for the main clause that would complete the thought, and the lines as quoted never supply it. An ellipsis then jumps us to a closing pair of lines, so we are reading a fragment of a longer poem. Even so, its central idea is plain. Human beings damage a natural world that deserves reverence, and that damage costs people something too.
The key words are chosen with care. "Profane" is a religious word, the opposite of sacred, and it quietly turns the landscape into a temple and the human hand into a trespasser. "Tainting" goes further than "damaging", since it suggests contamination, a stain that spreads. "Primaeval" makes the loss deeper still, because what is spoiled belongs to the world's earliest age and cannot easily be won back. There is a small sound-trick too: "man's" and "marred" share an initial letter, so the culprit and the damage sit close together in the line.
The rhyme scheme deepens this. The quatrain rhymes ABBA, with "hand" and "command" on the outside and "marred" and "debarred" inside. The outer pair sets man's hand against Nature's command, as if two rival powers were being introduced. The inner pair links cause and consequence, since the marring leads straight to the barring. The metre helps as well. The first line is short, at about eight syllables, and the next three stretch to a fuller ten. That clipped opening has the sound of an accusation, and the longer lines let the complaint unfold.
The middle lines are the knottiest. "And some few souls of the high bliss debarred / Which else obey her powerful command" bends ordinary word order to land on its rhyme. My reading is that a small number of people are shut out from a lofty happiness that would otherwise answer to Nature's call. "Debarred" is the language of exclusion, like being refused entry to a room you once belonged in. Nature is personified as a ruler whose "command" can be obeyed, so joy in the natural world becomes a kind of loyalty. The phrase "some few souls" suggests that only a sensitive minority feel this, though that is an inference from the wording rather than something the lines state.
The last two lines change the mood. "…mountain piles / That load in grandeur Cambria's emerald vales" turns from loss to abundance. Cambria is an old name for Wales. "Piles" can mean heaped masses of rock, and the word also carries a hint of great buildings. "Load" normally describes a burden, yet here the burden is "grandeur", so weight becomes a blessing. "Emerald" is both a colour and a jewel, which makes the valleys seem precious as well as green. The line is also longer and heavier than those before it, with its run of stressed syllables and its slow ending on "vales", so the verse itself feels loaded. Because this follows a poem titled "Farewell to North Devon", it is tempting to read a leave-taking from a place felt to be spoiled and a turn toward grander country. That is only an inference, since the extract never says so directly.
What makes the passage worth reading is how much it packs into six lines. It shows a Romantic habit of seeing nature as sacred and authoritative, with human intrusion as a kind of desecration. Shelley is one of the major English Romantic poets, and the vocabulary here (profane, primaeval, command, grandeur) is typical of that elevated, almost sermon-like manner. The idea underneath it also feels very current. Many of us now talk about landscapes being harmed by people, and here the same worry is put in the language of sin and sovereignty. The poem asks us to feel a loss, and to see that the people who lose out are those cut off from nature's "high bliss".