Most train sets are sold with an inexpensive power pack that is hooked up with two wires to the metal rails of the track. A lever or dial on the pack is used to supply power to the tracks to control the speed and direction of the locomotive.
By adding a radio control receiver in the wires between the power pack and the track, the power pack can be turned up full to provide the receiver with power. The receiver in turn will be used to supply power to the tracks to control the speed and direction of the locomotive.
The receiver however, does not have a manual lever or dial like the power pack. Instead it is controlled by a hand held transmitter, sometimes referred to as a throttle. When the appropriate buttons on the transmitter are pushed, information is passed through the air between the transmitter and receiver similar to the way two way radios exchange information. The receiver responds and controls the speed and direction of the locomotive. Now the operator is not tied to the power pack, and can follow their train to take pictures, throw switches, couple or uncouple cars, etc.