This is a shift in the poem, a moment where the speaker shifts his tone and his strategy.
Just like in the first line, he talks directly to the bird. (Remember that, in poetic terms, we call that an apostrophe.)
Here he uses the word "Sprite," (a fairy, a magical creature) to refer to the bird. This reminds us (just like in line 1), that the speaker isn't sure whether this bird is part of the natural or the supernatural world.
What sweet thoughts are thine:
The speaker wants to know what thoughts are behind this beautiful singing. He thinks of this bird as an artist, and he wants to know its secrets.
I have never heard Praise of love or wine That panted forth a flood of rapture so divine
The speaker is looping back to comparing the singing of the bird to poetry (just like he did in lines 36-40). Praising love or wine are ancient standard subjects for poems.
In all those poems, the speaker has never heard anything so full of joy and "rapture" as the song of the skylark.
Notice the word "flood." It seems like the skylark's song is always overflowing and flooding and sloshing all over the place. Again, the skylark is not just killing it with this song. It is way over-the-top killing it. It's almost as if it's a little too much for our speaker.