What we have here is the speaker comparing the shrill sound of the skylark's voice to the light that comes from a "silver sphere" in the sky.
Instead of just referring to beams of light, though, the speaker uses a metaphor, which represents the light as "keen" (sharp) arrows. To make things more complex, though, the metaphor is in turn locked into a simile. Following from line 20, the speaker describes the bird's "shrill delight" by saying that it is as shrill as the sharpness of these metaphorical arrows.
Whose intense lamp narrows In the white dawn clear
More clues here. It could be that the sphere in line 22 is most likely Venus, the morning star, which shines brightly and then fades away.
Again, there's a feeling that this is all a little overwhelming, even dangerous. This bird isn't just a calm little songbird. Its voice shoots out like blazing sharp arrows of light (or, in another metaphor, like beams from an "intense lamp").
We're kind of embarrassed to say this, but we think this bird is a little scary, what with all the "shrill delight" and "keen arrows" and what have you. (Check our "Best of the Web" section for a picture of a skylark—then you'll see why it's embarrassing to be scared of them.)
Until we hardly see, we feel that it is there.
This line is an echo of the simile in the last stanza (lines 18-20). Now it is Venus that is there, but almost impossible to see.
This is another way of making the bird seem almost-ghostlike. When the speaker calls it a "Spirit" in the beginning, that sets the tone for the whole poem. The idea of the spirit bird comes up over and over again.