This is the 20-inch D-Tap to 4-pin XLR Cable.
Just as the name suggests, this cable allows you to power a device that has an XLR4 connection via battery with D-Tap connection. It works with a wide variety of equipment ranging from camcorders to LED lights, and popular battery systems such as Anton Bauer Gold Mount, Switronix Powerbase, and V-mount batteries are compatible.