Idling is continuing the vehicle's engine to run when the vehicle is not in motion. This usually happens at a red light, waiting, or otherwise when the vehicle is stationary with the engine running.
It is wise decision to turn off the ignition if waiting for more than 10 seconds. Restarting the car does not burn more fuel. In fact, idling a car for 10 seconds wastes more fuel than starting the engine again. It has been proved that letting the engine idle uses more gas than starting and stopping.
Contemporary vehicles, when subjected to idling, consume more gas than they do for starting the engine. So, while the vehicle is parked for a short period of time, fuel is saved by turning the engine off and restarting it. The AAA says a quarter of a gallon of gas is used for every fifteen minutes of idling. So, doing this for five days a week burns $4 worth of gas.