On some development and prototyping boards, only pins or screw terminals are provided for connecting a power supply. This convenient adapter allows you to use a power supply that has a barrel jack connector. One end has screw terminals to connect wires to your board; the other has a 2.1mm × 5.5mm female barrel jack to connect to your power supply.
The markings on the plastic moulding assume that your power supply will be center-positive. Obviously, you can connect your source wires opposite to the markings if you have an outer-positive supply.