Fill light (including flash) is used to control shadow exposure.
When you have a light source that is not on the same angle as the camera, that light will likely give you both lit areas and shadow areas on a subject. For example, if the sun is above a person, it will likely cause shadows under their eyes, nose, chin etc.
Fill light is typically directed to the subject from an angle that is closer to the camera, or maybe on the opposite side from the main light...so it is likely to light up those shadow areas caused by the main light. For example, an on-camera flash will light up the shadows caused by the sun.
The brighter the fill light, the brighter those shadows will be. So by controlling the amount of fill light, you can control the ratio of bright areas to the shadow areas on your subject.