In profuse strains of unpremeditated art. (5)
It might kind of zoom by you on the first read, but Shelley is using this simple line to make a point about how he thinks the world works. He's suggesting that the skylark is making a kind of unplanned music ("unpremeditated art"). That's important, because it sets up an idea that runs through the rest of the poem. That idea is that you can compare the art of nature with the art of humans.
Like a Poet hidden In the light of thought, (36-37)
We've already talked about how the speaker uses a pretty big grab-bag of images to try to describe the song of the skylark. This one is extra-important though, because it brings back that connection between art and culture and the natural world. Basically, anytime a poet writes a line about a "Poet," it's time for us to pay attention. Eventually, the speaker is going to compare himself to the skylark, and this lays the groundwork for that comparison.
Chorus Hymeneal, Or triumphal chant, (66-67)
This is the kind of stuff that hip young poets talked about in the 1800s, but it's definitely not the kind of things we care about today. Maybe that's all the more reason for us to break it down here, though. Basically, the speaker is saying that these two kinds of human songs or poems (one for a wedding and the other for a military victory) are total junk when you stack them up next to the skylark's song.