An access point ...
http://tinyurl.com/qfy9hhh
It seems that quite a few access points only have one port. The reason I show this one is it has more than 1 port on the rear panel. You can get them cheaper, slower, faster it's all down to your budget.
When used with power line adapters it saves you running an ethernet cable around the house. One adapter which your router plugs into and another where your access point is. So in effect you get a similar wifi signal near the access point as you would near your router.
I'm afraid it's a bit of a minefield when you are not quite sure what to look for and everyone who is selling it seems to have the answer. I always go through the independent reviews when searching for hardware.
My neighbour put me onto these as his business is commercial telecoms and he say's these are what he would fit in a small office setup where the issue is a poor signal.
He also said to steer clear of wi-fi boosters ... in some cases they work in others they don't ... read the reviews.