This poem of course amazingly captures the spirit and majesty of the West Wind through a series of incredible descriptions, depicting the Wind as both "Destroyer and Preserver" before imprecating the wind to inspire the speaker to spread his thoughts and ideas through the world like the dead leaves that are blown by the wind, that will in turn bring life.
The quote in the second stanza that you refer to is part of a larger description that describes the wind's effect on clouds. A series of comparisons are made that will help us understand the description. Let us just recall what is said:
Thou on whose stream, 'mid the steep sky's commotion,
Loose clouds like earth's decaying leaves are shed,
Shook from the tangled boughs of Heaven and Ocean,
Angels of rain and lightning: there are spread
On the blue surface of thine aery surge,
Like the bright hair uplifted from the head
Of some fierce Maenad, even from the dim verge...
Thus we can see that your quote is part of a much larger chunk of this poem that describes the clouds of the "approaching storm" as "locks" on the head of "some fierce Maenad." The quote you pick out refers to the way that the wind blows these clouds across the blue sky.