When used in conjunction with Power over Ethernet compatible equipment such as an IP CCTV security camera, a midspan is the device used to combine power and data into a single cable. The device connects from your existing network to your camera, as well as to the mains power point, and provides power as required.
The way a midspan works is that whenever a device is connected to it, a small amount of power is transferred along the cable; not enough to power the device, but enough to power the port. The midspan will then send a short signal over the network to ask the device if it needs power. A Power over Ethernet enabled device will return a message saying that it does require power which the midspan will then supply. A device which does not support Power over Ethernet will not reply and thus will not receive any additional power. This means that you can never damage a device that does not support Power over Ethernet by plugging it into a midspan.
The power outputs from a standard (IEEE802.3af) Power over Ethernet Midspan is 48V at 350mA. Should a device require more than this, such as a PTZ camera, you need to look at HiPoE midspans (IEEE802.3at) which offer a higher current.