A fairy ring is a ring of mushrooms or stones on grass or in a forest. It is a place where fairies dance, or have danced. Fairies are known for their revelry and their love of dancing. Many nights are devoted to feasting and partying and dancing, and traces of these revels are left over in the form of these rings.
While this seems harmless and fun, fairy rings can be very dangerous to humans. If a human were to walk into a fairy ring there is a chance of that person being transported to the fairy realm, and once there there’s no telling if or when you might return home again.