Another Access Point, which can be configured as a "repeater".
An example: I want to use my laptop in the building across the street. Unfortunately on the first floor of that building the signal strength is typically about -83 dBm at best, and depending on obstructions (literally on where cars are parked between the two buildings!), might be more like -95 dBm. Well, -83 dBm isn't very good, and -95 dBm essentially isn't there.
So I put a repeater on the second floor of that building. That's not where I want use the laptop, but that is where the signal strength is good. The repeater gets a signal more like
-70 dBm now, and so does the laptop.