A linguistic imitation of reality. The second line of this brief poem is all image. We can imagine, we can see in our mind's eye the flower's stark contrast with the branch. Perhaps it has grown there. Perhaps it was blown there by the storm.
Images are always sensory.
They ground the poem's themes and ideas in real things. Nothing but image. Williams and Pound and other influential poets of their time joined together in a movement called Imagism. The Imagists argued for the primacy of image in poetry. To be really practical, what images can do in your poem is draw the reader in.