In plain English
PoE eliminates the need for a separate power outlet at every network device. A PoE-enabled switch sends both data and electricity down the same Ethernet cable, so an IP camera or Wi-Fi access point can be mounted wherever network cable can reach, without needing a power point nearby. This is especially useful for cameras positioned at ceiling height, in car parks, or in locations where running both power and data cabling would be difficult.
Full definition
PoE works by having a PoE-enabled switch inject a low-voltage DC current onto the same Cat5e or Cat6 cable carrying network data. The connected device, whether a camera, access point, or door controller, draws power from that cable rather than from a wall outlet. The IEEE 802.3af standard delivers up to 15.4 watts per port; 802.3at (PoE+) delivers up to 30 watts; 802.3bt (PoE++) goes up to 90 watts for higher-demand devices.
For physical security deployments, this changes where you can put cameras. A car park column sixty metres from the nearest power point is still reachable if you can run network cable there. Ceiling-mounted access points in a warehouse can go wherever coverage is needed, not wherever an electrician ran conduit in 1998. That flexibility typically reduces installation cost and, more importantly, means cameras cover the angles that actually matter rather than the angles that were convenient to cable.
The switch becomes a single point of management. Power cycling a locked-up camera means cutting PoE to that port from the switch interface, no ladder required. In a security context, centralised control also means switch logs can show when a device lost power, which is useful during an incident investigation. One thing to check: the total power budget of a PoE switch. A 24-port switch might support PoE on all ports but cap total power draw at 370 watts. If you connect 24 cameras that each need 20 watts, the math does not work.