A land of streams! some, like a downward smoke, Slow-dropping veils of thinnest lawn, did go
We start out this new stanza with a little exclamation, just like the word "Courage" in the first line. There's a little burst of excitement (marked by an exclamation point) to let us know the speaker is jazzed about all the streams in this place.
Also, can you see how that exclamation point makes a little break in the flow of the line? In poetry terms we call that a caesura.
To describe the streams, the speaker repeats his simile about "downward smoke" and then adds another metaphor, comparing the falling streams to a thin veil of fabric ("lawn" is a name for a type of fabric).
And some thro' wavering lights and shadows broke, Rolling a slumbrous sheet of foam below.
Some of the other streams of water in the valley are a little more powerful. They break out of light and shadow, and then fall down into a sheet of foam.
This image seems a little mysterious to us, a little hard to visualize. We think Tennyson does that on purpose, to increase the dream-like feel of this landscape.
The speaker also sneaks in a little more sleep language, describing the sheet of foam as "slumberous" (that just means sleepy).
We get some nice alliteration there too in "slumbrous sheet." That perks our ears up, even if the rest of us wants to go lie down somewhere and snooze.
They saw the gleaming river seaward flow From the inner land: far off, three mountain-tops,
Now we get back to the people we met in the first three lines of the poem. These guys, who have just showed up in this new landscape, can see a river flowing from the inside of the land toward the ocean ("seaward").
Then there's a little pause (or caesura) in line 15 and our focus shifts from the river to "three mountain-tops" far in the distance.
Clearly it's important to the speaker that we really be able to visualize this place… so focus, people.
Three silent pinnacles of aged snow, Stood sunset-flush'd: and, dew'd with showery drops,
In these lines, the speaker keeps describing the mountains we first heard about in line 15. He describes them standing "silent," covered in snow that never melts (that's why it's "aged" or old). In the moment he catches them, they're lit up and colored ("flush'd") by the setting sun. They're also surrounded by a kind of mist made up of "showery drops."
Honestly, this image gives us little chills. It's such a great combination of eternal stillness and beautiful freshness. The mountains and their snow are forever, but the dew and the sunset only last for a lovely instant—good stuff, if we do say so ourselves.
Up-clomb the shadowy pine above the woven copse.
One last nature image rounds out this stanza. The speaker imagines dark pine trees climbing up above the trees below them. A "copse" is a little grove of trees, and "woven" refers to the way the trees in the grove seem to grow together like woven fabric.
So this image is a contrast between the single dark pine tree that grows high up the mountain slopes and the trees that grow together farther down in the valley.