Please explain the following lines from "Ode to the West Wind."Thou on whose stream, 'mid the steep sky's commotion, 15 Loose clouds like earth's...

You have quoted the second stanza of this poem. It can
often be hard to follow the line of argument in such poems, but key to realise is that
in this stanza, the wind's effect on the clouds is being described, and thus we are
given a series of comparisons to explore this effect. Note the number of similes that
are employed, using the word "like." For example, in lines 16-17, we are told that the
clouds are shaken from heaven "like... decaying leaves... shed" from tree boughs.
Likewise, the clouds are described as being "Like the bright hair uplifted from the head
/ Of some fierce Maenad." A Maenad was a woman in Greek mythology who performs frenzied
dances in the worship of Dionysus, the god of wine. The effect of these comparisons are
to convey a kind of hysteria or sense of impending doom.

In
addition in this stanza, in line 28, the speaker of the poem addresses the wind
directly, imploring the wind to "hear" his words. The impact of this is to create an
invocation, as if the speaker were addressing a God, and therefore suggesting that
humans are powerless in the face of such might and majesty.