The wind sounds kind of like a superhero in Rossetti's poem, or at the very least a mythical sort of presence that passes by. At the same time though, the speaker describes the wind in the same sort of natural, no frills way that it exists in real life. Maybe the wind is just naturally mythical and mysterious.
Title: It's in the title and in the refrain. Even though we can't see the wind in the poem, just like we can't see it in real life, we know it's there. So it's invisible but we can still feel it all over the poem.
Lines 3-4: When those leaves are "trembling," you know the invisible wind is passing through. So we can't see the wind, but we can see its effect on other things like the trees, which proves that the wind is real.
Lines 7-8: The trees even "bow down their heads," which gives us a sense of reverence and respect that the trees appear to be offering the wind. They're not fighting against the force of the wind but allow it to pass by in its natural and invisible way.