"Resistance" Makes Heat
There are at least four ways that electricity can make heat. The one that we'll cover here is resistance heating. (The others are: dielectric heating, where the lines of force of an electrostatic field pass through a non-conductive material and heat it; the heat pump, which is a refrigerator in reverse; and electronic heating, which uses high frequency waves similar to radio waves to create high speed movement of the molecules or tiny particles which rub together to make heat.)
Resistance heating occurs because every conductor of electricity opposes the flow of current through it. Some conductors resist more than others. When they do, a certain amount of warming takes place. The more resistance that is offered, the more heating there is.
Some materials, like silver, copper, and aluminum, offer little resistance. We say they are good conductors.
Other materials, like iron, offer more resistance. They are still conductors, but not as good as the others mentioned.
The size of the conductor, and its length are the other two things that affect its resistance. The smaller it is, the greater its resistance. Also, the longer it is, the greater its resistance. Therefore, when we only want to move electricity from place to place, we want relatively large, "good" conductors. Here, we do not want to make heat. In fact, we want to avoid it, because too much heat in the wrong place can cause a fire.
But when we want heat, we choose relatively small, "poor" conductors, and the more heat we want, the longer they must be. If you will think of the filament inside a lamp bulb; you may recall that it is a very fine wire, coiled so as to get a maximum length, and made of tungsten which has a high resistance.
Because of all these factors, this filament glows at a white heat, and is a source of both light and heat.