Ethernet (your computer network) only uses 2 pairs of wires. Cat5 cable provides 4 pairs, leaving 2 pairs leftover. You can use those two extra pairs to transmit power (in small amounts) to power remote equipment such as another router, switch or a bridge.
This additional capability is not provided by default. This article provides clear instruction on setting up your own Power Over Ethernet (PoE) network, including making custom adapters for injecting & extracting the power.