For the VOY episode with a similar title, please see "Spirit Folk".
"Spirit folk" was an old Earth phrase used to describe individuals with seemingly supernatural powers.
In 2376, the continuous use of the Paris 042 holoprogram, based on the town of Fair Haven, on USS Voyager caused the algorithms designed to keep holodeck characters oblivious to anything outside the program's parameters to break down. When the characters witnessed the Voyager crew manipulating the holodeck program to their will, they began to believe that the Voyager crew were spirit folk, and attempted to violently put an end to their activities. (VOY: "Spirit Folk")