It’s a 3/4″ to 1/2″ reducer bushing threaded to a 1/2″ to 1/4″ reducer bushing threaded to a 1/4″ air hose quick connect.
Attach it to a garden hose and set your air compressor regulator to 50 psi.
Blows the water out of hose on a reel.
Great for getting a hose on a reel ready for movers to take it.
Also probably good for winterizing hose.
About $6 worth of parts at Lowe’s.