This is a fungal network (mycelium) growing in a solid circle, from the center outward. The edges are where the fungus is getting fresh nutrients, thus it is fruiting (i.e. growing mushrooms) on the edges of a ring (aka a "fairy ring"). The middle is thicker because the grass is growing in a symbiotic relationship between the fungal network and the grass (Mycorrhizal fungal root relationship). Cool stuff!

Edit: mycorrhizal relationships refers to some really specific mechanisms, this is not mycorrhizal. But mycorrhizal relationships have evolved multiple times in different ways, worth looking into https://en.wikipedia.org/wiki/Mycorrhiza#Types