The speaker is a spirit, and the poem is their message to someone still living. At first they felt the two of them couldn't be further apart, and they drifted into unseen places that people believe in but that, the speaker admits, don't really exist. Being cut off in that "semi-existence" became unbearable, so they've come back.
What they offer is quiet company. The speaker takes a kind of joy in being everywhere around the living person, haunting their world gently rather than frightening it. Animals seem to look back with "a certain knowing," a stray sound in the wind might carry a hint of a voice, and a flicker at the edge of vision could be a phantom glimpse.
The mood is tender, hushed, and a little eerie, but mostly comforting. The short last line, "I am still here," lands like a whispered reassurance, as if the speaker wants the listener to know they haven't been lost.