Definition: In radio controlled vehicles, the throttle controls the flow of fuel to the engine or power to the motor. Throttle directly affects the speed of the RC vehicle. More throttle equals more power and more speed. Less throttle slows down and/or stops the RC.
In pistol grip transmitters, the "trigger" controls the throttle. On stick controllers, the throttle is typically controlled by the forward / backward (up/down) movement of the left stick, although different stick positions are possible.
With some RC aircraft applying more throttle can also be the primary means of increasing lift for take-offs or climbing. Applying less throttle decreases lift for descents and landings.
On all but some basic RC toys, the throttle as well as other controls are proportional -- the amount of throttle (power/speed) is directly proportional to how much or how little you move the stick, trigger switch, slider, or other throttle control on the transmitter.
Examples:
Even on an electric RC, when you 'give it the gas' you're increasing the throttle. To slow down or stop you release the throttle -- pull back on the stick or lesson pressure on the trigger switch.