It helps to understand the purpose of every different light involved in basic lighting photography.
They are known as basics since they work, and all photographers must understand them meticulously.
It's main function is to offer the general lighting.
This is the type of light that normally use and read for your exposure mode.
It does not imply that you should be using a studio strobe to be a central source.
It can be your camera flash, ambient light or perhaps a reflector that bounces illumination to the face or subject.
You can make you subject look 3 dimension as compared to flat and dull with your main light turned off on the side.
This will create opposing shadows on the side of the face or subject.
It will rely on the power of your main light to make the shadow area extremely dark and concentrated.
Dark and concentrated shadows are not recommended for most portraits.
You might want to lighten the shadow so that the details of the face or subject can be seen.
The fill light can be an added studio light, on camera flash, reflector bouncing light, or perhaps ambient light hooked on the shadow areas.
The concentration of the fill light that is comparative to the main light will establish the amount and depth of the shadows.
The typical rule of thumb is making the fill light at the about half the depth of the main light.
The separation or background light is the third light.
You can use just any light sources that are readily available.
This is used to take the subject apart from the background.
The light can be turned away from the subject and straight to the background.
It can also be turned towards the subject making a rim light.
The light can also be placed higher or above the subject to create hair light.