A power injector POE of IEEE 802.3af standard is implemented in a form of an adapter that can be plugged into an electric outlet. It supports midspan plugin in a gap of an Ethernet cable. On an input of an injector an Ethernet is connected without POE, on an output there is an Ethernet with POE support. It provides up to 15.4 Watts to feed other devices by a POE technology.
POE (Power over Ethernet) is a technology of sending data and electric power through a standard twisted pair in an Ethernet network. For example, if your video camera supports POE, it is enough to connect an Ethernet cable to plug it in, and there is no need to plug it to a 220V, as a camera will power through an Ethernet network. The technology is widely used in IP telephony, wireless network access points, IP cameras, network hubs, IP weather stations. Using POE power supply can be critically important when connecting a separate power supply is undesirable or impossible. For example, in the installation place there is no external power outlet or an external power supply is unstable. You can read more detailed information about a POE technology here: Power over Ethernet - Wikipedia.
A POE injector can be used together with NP-SM4 POE switches that support POE power supply and allow to build branched networks with POE, transmitting power one another via an Ethernet cable: