What exactly is desired when asked to explain what a metaphor means?I have to give examples of metaphors in "Ode to the West Wind" and explain what...

Concerning your question on Shelley's "Ode to the West
Wind," to analyze the meaning of a metaphor you just break it down and tell what is
being described (the tenor) and what is being used to describe it (the
vehicle).

First, I believe the line you quote is actually
personification, rather than metaphor.  The wind is given breath
and Autumn is given being.  Don't feel badly about mixing the two
up.  Personification also involves comparison, as does metaphor.  Metaphor, in fact, is
actually an umbrella term that, when used broadly, covers all figurative language, since
all figurative language uses comparison. 

But for your
purposes we'll stick to the traditional definition of
metaphor. 

In line 54 of the poem, Shelley
writes:

I fall
upon the thorns of
life!...

This is a metaphor
in which falling on or meeting or facing the difficulties of life (the tenor) are
compared with falling on the thorns of a bush (the vehicle). 
Thorns or thorns of life form the metaphor. 
And experiencing the difficulties of life is like falling on
thorns. 

If you need more than the above for your writing
assignment, simply elaborate on the ways in which the two parts of the metaphor are
alike.  Or, to put it another way, elaborate on how experiencing life's difficulties is
like falling on a thorn bush.