What do the lines "sound of vernal showers/On the twinkling grass/Rain-awaken'd flowers,/All that ever was/joyous, and clear and fresh, thy music doth surpass" mean?
In this Romantic poem, the speaker offers praise to one of Nature's beautiful creatures, the skylark, that sings while in flight. The particular stanza you cite could be explicated in this way:
the sound of spring showers/on rain-sprinkled grass/flowers refreshed by rain/and everything that was ever "joyous, clear and fresh", pales in comparison to the song of the skylark
The poem is an ode, and what that means is that the speaker is directly addressing the subject of the poem, the skylark. Odes are intended to praise the entity that they address. The speaker respects and admires the bird and in the stanza you have asked about, essentially tells that bird that its song is superior to other beauties found in the natural world.