The ring-like skirt are a membrane that surrounds the stem of the fruiting body. In fungi such as Button Mushrooms and Fly Amanitas, the ring is a remnant of the stem that bursts when the cap falls off.
Sources
Britannica – hymenium – fungus tissue 
Unsplash – Photo of mushrooms
April 2023, TÄNKOM | Revised February 2024 RETHINK
