A fairy ring is a naturally occurring arc of mushrooms that usually appear in forests, but may also grow in grasslands. While the name of this phenomenon and lore behind it vary by culture, the most widely recognized tale indicates that fairy rings were created by a circle of dancing fairies. Numerous tales caution against entering these rings for fear of being cursed to die at a young age, being luring away into the fairy realm etc.
Modern science now recognizes this phenomenon as the result of fungus (the most common of which is Marasmius oreades). The body, or mycelium, of the fungus grows underground in an outward circle and as it grows it steals nutrients from nearby grass. Mushrooms spring up around the outer edge of the mycelium. As the ring grows the fungus releases a chemical to break down outward organic matter, allowing for a surplus of nutrient the mycelium can use when it reaches it. This accounts for the darker, taller, thicker grass often seen with fairy rings.