I'm still learning about lighting and during my reading I've come across mention of using a soft fill light.
I can't quite envisage what effect this has. I always thought the key light determined the shape and harshness of the shadows and a fill light just altered their brightness. What effect does a softer fill light have?
The fill light "alters the brightness" of the shadows of the main light (reduces contrast). The main light is from an off camera angle, and may be large, and may make soft shadows, but it has direction, which when seen from the camera angle will see its shadows (creating the shadows are its purpose; perhaps vague tonal gradients, but interesting shadows). The fill light, from near camera lens axis, illuminates those shadows without adding more of its own.
The fill, if from camera lens axis angle, is soft by definition, in that it does not make shadows at all (not that the lens can see). But it is made large and soft, because it cannot be precisely on the lens axis.