Power over Ethernet or PoE technology describes a system to pass electrical power safely, along with data, on Ethernet cabling.
PoE cable basically "injects" power into an ethernet cable, This way you are able to use one cable for power and data at the same time. This is great when where AC power would be inconvenient, expensive or infeasible to supply.
Each cable has a female RJ45 on one side which is adapted to a male RJ45 and 5.5x2.1mm barrel jack connector on the other side. The 'injector' cable has a female barrel jack connector and the 'splitter' cable has a male barrel jack.
Simply attach the cables at each end of your network cable, plug the male barrel jack from the splitter into your target device and plug your power adapter into the injector. Now you can use one cable to provide both power and data for your Ethernet enabled device.