The product is made by On‑Stage and it converts a 5/8″ male thread (common on standard mic stands in North America) to a 3/8″ female thread (often found on European‑style boom arms or accessories). 
Model “MA‑200” is specifically the 5/8″ male to 3/8″ female variant. 
It’s essentially a simple, inexpensive little adapter that lets you mix and match mic stand hardware across different standards.