A normal low-wattage power connection for a computer has a cable from the computer to an external power supply brick, and then another cable from that to the wall. A computer powered via PoE does *exactly the same thing*, except that the cable between the computer to the external power supply brick happens to be the same cable that is being used for data. At the other end of the data connection is a device with a power supply which plugs into the wall. You can’t use PoE if you don’t have something at the far end of the data connection providing the power; it’s not using some sort of latent power found in all ethernet cables or pulling electricity out of the air along the cable run or anything magical like that.

Power over ethernet is handy (especially for things like IP-based desk phones, outdoor wireless routers, and so forth), but there’s nothing particularly spectacular going on: the only development is that the power cable and the data cable are both enclosed in the same insulation and share a connector.