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 they mean. I believe that, for example, line 1 where it says "O wild West Wind, thou breath of Autumn's being," is a metaphor comparing autumn to a living creature. Yet, is that what they are wanting when they ask what the metaphor means? I'd appreciate your opinion.
In any type of question of clarification, I would make sure that you are clear with what your instructor or teacher is asking. This might require you to open a dialogue about the topic with them. Outside of that, I would suggest that a metaphor in poetry is a type of extended comparison that allows the poet's message to be clearly articulated through the convergence of multiple images. For example, in Shelley's poem, the desire for poetic immortality is linked to natural phenomena, such as the changing of the seasons or the power of the winds. In this respect, Shelley is seeking to bring together natural experiences and his own subjective understanding of his poetic sensibilities.
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.