This means that when buying an RTF quadcopter, you will receive all the things that are required to fly. So this includes the actual drone, a flight battery, and also a R/C radio control. In most situations a battery charger is also included. Usually some extra AA batteries are still required for your R/C controller.
Also in the case of FPV quadcopters, you will also be expected to add your own FPV headset to view the live video feed. Although there are a few combo packs that are sold that include everything, its always best to check product description for more details, or ask the local shop.