The speaker feels disconnected from everything, as if they are a cloud of fog while everyone else is a ghost. The metaphor lets them describe a loneliness that feels both physical and emotional. Fog drifts, can't be held, and keeps its distance even when it doesn't want to. The speaker has learned to accept that gap, but the poem insists that caring doesn't fade just because someone is far away.
Most of the poem is addressed to a particular "she," and the speaker frames their habits as "lessons" for her: I drift, and I still care. The hollow arms reaching out are the most moving image. The speaker doesn't need her to reach back; trying is enough. Still, the ending is blunt and sad: the fog loves the ghost, and the ghost won't love the fog back. The mood is quiet, tender, and heavy with resignation, with a small thread of stubborn hope running through it.