PoE, used by the digital wall clocks stands for Power over Ethernet. It is a way of simplifying the the installation of networkable devices. Essentially, the device takes its power from the CAT5 network cable, eliminating the need for a mains outlet at the point of location of the digital wall clocks.
If your network segment is PoE enabled, all you need to do is to plug the digital wall clock onto your network and it will power-up automatically.
If your network is not PoE enabled, you will require a low-cost midspan-convertor. A midspan converter is a small power supply that injects 48V DC onto the CAT5 cable to power the PoE device. Midspan converters can be sourced in a variety of formats, either single or multiport.
PoE is a standard way of implementing network powered divices and is extensively used by IP telephone networks.