A wisp is a thin bit or thread of something. Even after you put out a campfire, there may be little wisps of smoke in the air above it.
Tiny pieces of hair escaping your ponytail are wisps, and tufts of clouds or fog are also wisps. A more figurative kind of wisp, like a wisp of memory or sadness, is just the barest snippet of emotion. A wisp of a child is a tiny girl or boy, and a wisp of hay is used to dry a horse — this is actually the original, 14th century meaning of wisp, related to the Swedish visp, "bundle of hay."