You plug one unit into an outlet close to your router and connect it to your router with an ethernet cable. You plug the other unit in in any other room in your house and connect it with ethernet to a computer, a second router, or any other device that has an ethernet port.
I do this at my place that we're renting. It works well, but agree that the house wiring will be the limiting factor.
The last place we were in had shoddy wiring and it did struggled. Speed was like 30-50mbs versus the 1200-1500 mbs we get now (maximum is 2000 with the kit we have).
Did you test it? Don't blindly trust the number on the box. My distrust of Powerline is based on testing different TP-Link sets at different homes; the speed was almost always slower than Wi-Fi.
I guess there are various factors that can affect this; I'm not an electrician but I assume that the way your home's power grid is set up might make a difference.