Powerline adaptors are a means to increasing a network's coverage when it is difficult or inconvenient to do so with Ethernet cables or Wi-Fi.
They are usually supplied in pairs and work by simply plugging them into two mains sockets at home, then connecting their Ethernet ports to the network and a device that would like to be on the network. The high frequency data signals are filtered away from the very low mains frequency voltage.
As an example, you might want to use your laptop at the top of the garden, but your wireless router is at the front of the house and the signal is too weak. You could plug a powerline adaptor into the mains near the wireless router then plug its Ethernet port into the router too. Now run a mains extension lead to the end of the garden and plug the other adaptor into that, then your laptop into the Ethernet port. You now have a wired connection to your network.
Modern powerline adaptors give very good speeds and some are able to be password protected to prevent hacking. However, care should be taken in a shared environment e.g. office blocks, where multiple adaptors could be used to inadvertently connect disparate networks. Stickman does not recommend the use of these at sites they service; they can cause huge problems in these situations and the IT infrastructure in place should make them unnecessary anyway.