Many tools have a fill light option but some may call it something different. Fill light does what the name might imply if you are used to using a camera flash to fill in shadows: it brings up the shadows a bit without affecting the highlights too much
I usually explain it in this fashion. Fill light has the property of reducing contrast as you add more + Fill. Conversely, minus fill light darkens and increasing contrast. That would be the difference between Brightness and fill light.
Hope this helps.
Fred