Here is the way to describe it so that anyone can instantly understand it. Suppose you have only one light and you are in a dark room. No matter where you put that light, it is the main light for your subject. I think everyone will agree with that. Now look at the light that falls on the subject. Ask, do I need to add another light? What will the light do? Will it soften shadows? Then it is a fill light. Will it rim the hair? Then it is a hair light --- and so forth. The whole scene is dominated by that main light. If you set up other lights and then take away the main light, you would see a total transformation of the scene. Also keep in mind that it is not necessarily the light that lights the majority of the subject. The main light is also the only light that should cast visible shadows.
"The map is not the Territory"