The poem opens with the title that asks, "Who has seen the wind?" The speaker informs us that neither she nor anyone else has ever seen the wind. But we do see the leaves "trembling," which informs us that the wind is passing through.
The speaker then repeats the same question in the following stanza. The answer remains the same that no one has seen the wind, but when the trees "bow down their heads," we again realize that the wind is passing by.