Something wraithlike looks like a shadow or a ghost. People walking on a foggy beach could look wraithlike from far away.
Things appearing vague and shapeless can be described as wraithlike: a play might feature wraithlike dancers moving on a darkened stage, and the haunted house at the state fair might feature wraithlike creatures gliding around and screeching. This adjective comes from the noun wraith, or "ghost," which is Scottish in origin, possibly stemming from an Old Norse root that means "guardian angel."