This cable is 30cm long. On one end is the Lego NXT male plug, and on the other end is the Power Functions® 4 stud connector.
This cable allows you to drive the L12-NXT Linear Servo manually using your Power Functions® hardware. It will function with the switch, battery pack and IR remote control.
You can also use this cable to drive the NXT Servos made by the LEGO company, with your power functions equipment.