Through subtle imagery, the speaker suggests that magic lies just beneath the skin of the natural world.
The poem opens on a "Murmur in the Trees" that the speaker is wise enough to take "note" of: there's something a little strange about it. You might think it's the wind, but if you're paying careful attention, you'll find it's "not loud enough" for that—a distinction so fine that you really have to know what you're listening for to grasp that you're hearing voices, not breezes.
Similarly, the "long—long Yellow" the speaker sees stretching across the "Lawn" vividly evokes rich late-afternoon sunlight falling over grass. But again, the speaker knows that there's something special about this light. Readers might here imagine the way the world looks when the light is low in summer and autumn. The speaker picks up on the gold-lit, enchanted feeling of those fleeting times to suggest that the "magic hour" really is magic. This image also suggests that magic appears in in-between times and places, like the hours when day and night shade into each other.
When supernatural beings finally put in a direct appearance, they introduce themselves through a "Hubbub—as of feet," a ruckus that isn't "audible" in quite the same way ordinary footsteps are; it sounds "dapperer—more sweet" than human plodding. Notice the way the speaker plays with onomatopoeia here: the word "dapperer" itself sounds like the patter of light feet over leaves.
All of this imagery is shifty, elusive, just on the edge of perception; even when the speaker hears fairies going about their business, the sounds they make simply aren't "audible" in the same way human noises are. Catching a glimpse of the enchanted world, the poem suggests, demands careful, patient attention, a respect for what you can sense out on the very boundaries of your understanding.