Some quadcopter kits do not come ready to fly (RTF) and they will need something extra to complete it. Cheap drones are ready to fly, meaning you simply charge the battery install the controller batteries and with some simple calibration, you are good to get airborne.
Other models described as BNF, which mean Bind, and Fly need you to buy a controller first and “bind” it to the plane in order to be able to fly. In this case, you need to make sure that the two are using the same channel and manufacturer protocol.
Finally, ARF stands for Almost Ready to Fly and the “almost” part is quite up for interpretation; the plane kit may come in separated pieces, which need assembly, or it might even leave out important things like the motors. Therefore, you really NEED to read what you are getting before hitting the buy button.