The modem/router is what connects your house to the internet - it's either plugged into the phoneline or to cable. If you are using the PS3 in the same room as that box, there should be no problem but if you are relying on it picking up signals through brick walls you may need something to boost it.
There are several ways you can do this. One is to wire the house, from the router to wherever the PS3 is, using an ethernet cable. Another, more flexible, way is to use devices called Home Plug AV. You must use at least two of these to make them work. You plug one into an electrical socket near the router and connect it to the router using an ethernet cable. You plug the other one into an electrical socket in the room where the PS3 is, and that's it - your internet signal is being taken round the house using the ordinary wiring system. Seriously. It's magic. Or rather, it's electric.
