A metaphor makes a comparison, and in doing so shapes our perception. If we say, "Time is a river," we're noting a certain similarity between the two. Yet we know they aren't identical. We may mean that time is fluid, has currents and eddies, empties into some vast ocean, but not that it's composed of water. If we say, "Time is a stone," we may mean it's silent, still, indifferent, but not that it's a mineral.
Because of this power to shape perception, metaphors are the very essence of poetry. While beginning poets may see metaphors as ornamental or decorative, more experienced poets use them structurally, sometimes extending and exploring a single metaphor throughout an entire poem, as Walt Whitman does in "A Noiseless, Patient Spider," or Seamus Heaney in "Digging."
You might also use metaphor to clarify central concepts or to connect parts of a poem. Notice how William Carlos Williams uses snake, flower, and stone metaphors in "A Sort of a Song."
Or you could think metaphorically about your poem's overall design:
This poem will be a thunderstorm: first a sunny sky with a few light clouds and some stirring of leaves, then a sudden drop in air pressure as the clouds join and build into thunderheads--driving rain, thunderclaps, lightning--brief but intense and frightening until the storm blows off east, leaving behind a few broken tree limbs, water flowing down the streets, the grass green and vibrant, the air moist and cool.
Write up an extended metaphor like the example above, describing a poem you'd like to write or are currently working on. Next, free write a few minutes, incorporating related images and details whenever appropriate. Then, fashion your freewrite into a poem of ten lines or less.