Behind a router means that the computer being talked about is connected via router. If your router is used to connect to the internet, then both PCs are behind a router.
If one PC connects to the Internet directly and is connected to the other one via router, then only one is behind a router, as viewed from the internet. But both are behind a router as regards each other.
Basically, if you have to go through a router to get to the PC from where you are referring, then the PC is behind a router.
It means that the PC is protected by a hardware layer (the router).