This adapter allows you to connect a barrel jack connector to bare wires. One end has screw terminals and the other has a 2.1mm × 5.5mm male barrel jack. If you need to connect bare wires to a barrel jack on an Arduino, BeagleBone or other development board with that size barrel plug, this can be a simple solution.
The markings on the plastic moulding assume that your power jack will be center-positive. Obviously, you can connect your source wires opposite to the markings if you have an outer-positive jack.