The part of this poem that you are looking for comes in the eighth stanza, where Shelley compares how unknowable the skylark is to a poet writing his poetry. This is of course the first of several stanzas where Shelley tries to find a suitable comparison to describe the mystery and poignant beauty of the skylark and its song. Let us consider how he does this:
What thou art we know not;
What is most like thee?
From rainbow clouds there flow not
Drops so bright to see
As from thy presence showers a rain of melody.
Like a Poet hidden
In the light of thought,
Singing hymns unbidden,
Till the world is wrought
To sympathy with hopes and fears it heeded not:
Shelley is struggling to describe the beauty of the skylark, but he finds a suitable comparison in the poet, who is "hidden / In the light of thought." The poet, like the skylark, "sings hymns unbidden" until the world is awakened to the "sympathy and hopes and fears" that it was previously unaware of, but now, thanks to the poetry of the poet, has recognised. The skylark's song therefore helps us become "more" human in recognising more of our own condition and being aware of our emotional situation.
Hail to thee, blithe spirit!
Bird thou never wert-
That from heaven or near it
Pourest thy full heart
In profuse strains of unpremeditated art.
That first stanza of Shelley's poem calls the bird's song "unpremeditated art." In other words, human poets have to premeditate, have to think about what they're going to write. But the skylark's music pours out naturally without the bird having to think about it ahead of time.
Teach us, sprite or bird,
What sweet thoughts are thine:
I have never heard
Praise of love or wine
That panted forth a flood of rapture so divine.
In that stanza (which comes much later in the poem, I've skipped over quite a few stanzas) "Praise of love or wine" means "poems that human artists have written praising the joys of love and the pleasures of wine." The speaker or this poem, the human poet addressing the skylark, is saying, "The joy and pleasure that you pour forth in your song is greater than any expression of joy and pleasure that I have ever come across in any poem by a human author."
Or triumphal chant,
Match'd with thine would be all
But an empty vaunt-
A thing wherein we feel there is some hidden want.
A "Chorus hymeneal" is a poem that celebrates a wedding. A "triumphal chant" is a poem that celebrates a military victory. Shelley's poem is saying that the joy expressed in a wedding poem or a victory poem sounds like "But an empty vaunt" (just a meaningless boast) compared to the skylark's song. A "hidden want" is some lack, some shortcoming that can be felt but can't quite be defined. So the last line of that stanza is saying that even the most celebratory human poetry seems to be missing something when measured against ("Match'd with") the bird's singing.
Teach me half the gladness
That thy brain must know;
Such harmonious madness
From my lips would flow,
The world should listen then, as I am listening now.
In the final stanza of the poem, the speaker says, "If I were filled with even half as much beauty and magic as you seem to be filled with, I would be able to create poetry that would grab people's attention and blow people's minds as much as your song is affecting me right this minute."
There are other passages in the poem that talk about how much more wonderful the skylark's song is than any poem ever written by a human being. I've pointed out some of the highlights, but you can find more