This line sets the mood for the whole opening part of the poem. It starts us out feeling sort of peaceful. After all, what's more relaxing than a calm ocean at night? In a way, this is kind of a trick, because this poem is going to wind up being anything but calm. Arnold hides his bigger themes behind these pretty images. Then again, lots of artists use this technique—start small, then go epic. Plenty of the great rock ballads work this way, right?
Where the sea meets the moon-blanched land, (8)
We singled this one out just because we think it's lovely. It gives us such a great sense of place. We can just imagine that vivid, gorgeous line between the land and the sea. This poem is so much about the imagery of water, so it makes sense that Arnold takes extra care to make us see the line of the coast.
Listen! you hear the grating roar Of pebbles which the waves draw back, and fling, (9-10)
Here's where things start to shift in this poem, where the images of nature turn kind of ugly. We think it's all riding on that word "grating." Up to this point, the images of nature are harmonious and beautiful sounding. Then, all of a sudden, we get this ugly grating sound. It's one of the many ways that Arnold slips sad and grim elements into this poem.
Sophocles long ago Heard it on the Ægean, and it brought (15-16)
This line connects the speaker to the distant past, and it uses the natural world as a bridge to make that possible. The speaker imagines, essentially, that Sophocles was inspired by the same natural forces that he was, that listening to the waves brought tragic thoughts into both of their minds.
Retreating, to the breath Of the night-wind, down the vast edges drear (26-27)
We think this line is beautiful, but also a little creepy and lonely. Nighttime and the sounds of nature can be wonderful or threatening, depending on how a writer presents them. It's funny how a single word can make all the difference. The "night air" sounds lovely and calm, but then "night-wind" sounds a little sinister (at least in this case). It fills our heads with images of bleak darkness and howling wind, like you might find in a horror movie.