PoE, or Power Over Ethernet, is a way to provide electrical power to your wall or ceiling mounted access points using only the network cable. This way there's no need to worry about where to plug it in.
There are typically 3 types of power ratings that you will come across:
24V. the lowest typically used by UniFi AC Lite units, for example, 48V and the higher power 54V typically used by heavyweight devices such as longer range/higher density APs, switches and so on.
The PoE connects to your internet gateway/router (using the LAN socket) and also to the power source. The Access Point connects directly (using the PoE socket)
When using multiple APs, you don't need to purchase many PoE injectors. Instead you would typically install a PoE switch which behaves in a similar way to an injector, but can provide network and power connections to multiple APs.
A UniFi ToughSwitch
The UniFi ToughSwitch, for example, powers up to 5 devices on a single power source, making venue cabling far simpler, especially for ceiling-mounted access points or other awkward locations.
TIP: Always use vendor-specific PoE switches. If for example you connect 3 UniFi AP AC Lite units to a D-Link PoE, sometimes there's only enough power to run one of them at a time!