Essentially, the speaker is telling us that this skylark is actually a supernatural being and never was just a bird. This is kind of an unexpected way to start off a poem that's supposed to be about a bird. It tips us off, right from the beginning, that this isn't really a poem about nature, at least not in the literal sense. It's about emotion and spirit and purity and a whole list of other big fancy ideas. In a way, this little brown bird is just a jumping-off place.
In the golden lightning Of the sunken sun, (11-12)
This poem is just packed with intense images of nature. Phrases like "golden lightning" communicate the kind of extreme, almost overwhelming imagery that Shelley and his fellow Romantic poets were famous for. Lines like this make us feel like we're being washed over with a sea of color. Shelley turns the sort of simple, peaceful idea of sunset into a symphony of passion and joy and excitement.
Like a glow-worm golden (46)
We think there's something really cute about putting the glow-worm in this poem. Fireflies are great, but there's nothing huge or exciting or splashy about them. In fact, they seem kind of simple and quiet, although they are definitely magical in their own way. Our speaker's passion for nature runs from the really big stuff (the sun and moon and stars) all the way down to a little bug hidden away in a damp little valley.
Like a rose embower'd In its own green leaves, (51-52)
The speaker throws one image after another at us, trying to get us to understand what's so fabulous and mysterious about the skylark. In this case, he uses a pretty standard (maybe even clichéd) poetic image. This rose hides away like the skylark, but at the same time it releases its scent out onto the wind just like the skylark gives off its song. All these comparisons don't really help us to understand what this bird sounds like, but they might just give us a sense of what it feels like for the speaker to hear its enchanting song.
Sound of vernal showers On the twinkling grass, (56-57)
Aw. Another lovely image! We could talk about how this symbolizes the speaker's emotional awakening in the natural world, but we think it's mostly just one more element in the symphony that Shelley is weaving together here. We just love how alive everything feels in this poem, as if the whole world had electricity running and sparking through it. Even the grass is twinkling.