A midspan like the BT-1-55v30w is a simple device to add on to a network port, and the purpose is to add PoE using a wide range of input power to any network.
With this DIN rail mounted device, simply mount it on a DIN rale near the networking switch you'd like to pair it with. Then, using a short Ethernet patch cable from the switch's data port to the LAN port on the PoE midspan. Then the POE port adjacent to the LAN port will now have that IEEE 802.3at (30 watts) power budget and up to gigabit pass through data. Finally, connect the DC power supply to the Phoenix style connector on the top.
The BT-1-55v30w is also surge protected, so, while it cannot be mounted outdoors by itself, you can connect it to outdoor devices with confidence. In order to make the surge protection fully functional, you will need to properly ground the connector on the side of the device.
Not sure what grounding is? Panduit has some great graphics around grounding.