When you have your router in one room, and need to be able to plug something into it that is in another room, you can either run a cable between the two rooms (which is the best thing to do), or when that isn’t practical, you can use a pair of “Powerline adapters” to achieve nearly the same thing.
Powerline (sometimes referred to as PLC or Homeplug) adapters take a network cable from your router, and then connect into a mains power socket. A second Powerline adapter in another room is the connected to a second mains power socket, and you connect a second network cable between it and your device (computer, switch, wireless access point, set-top box etc). The electricity mains cables in the walls of your building carry the network data signal for you – meaning you still have a wired connection… only without the wire.
The original generation of Powerline adapters were marketed as being capable of a speed of “85 Mbps”, and today there are devices that claim 200 Mbps, 300 Mbps, 500 Mbps, 600 Mbps, and 1000 Mbps.
The speeds they give are nearly pure fantasy – you’ll never get 200 Mbps, nevermind 500 Mbps from them, but the higher numbered devices are faster than the lower ones.
They should all work together, but in practice, you really ought to try to have exactly matching devices (so don’t mix different manufacturers or speeds if you can help it).
TP-Link 600 Mbps Powerline on Amazon – around £20
TP-Link 600 Mbps Powerline at Argos – around £23
TP-Link 600 Mbps Powerline at Currys – around £25
TP-Link 500 Mbps Powerline at Maplin – around £30