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.