As there is sodium chloride, or simply salt, floating around, there are negative chloride ions and positive sodium ions. The red connector on the car battery is a positive electrode (that’s just a fancy way of saying terminal). Here, negatively charged electrons are stolen away from the chloride ions (since they have an influx of them), resulting in the formation of chlorine gas.
A controlled experiment is one where you compare the results of two different experimental situations, where one is the controlled condition and the other is the experimental condition. What highlights controlled experiments though, is that in the experimental condition, you must change only one variable to obtain reliable and accurate results. In the experiment described in the post, that’s whether or not you put the candle to your ear.