Fill flash is mostly the lighting you use in an outside photoshoot to brighten deep shadow areas. It lights up  a subject while having a non-sunny kind of day (dark background).

Photographers usually used this technique on sunny days because it gives them a way better outcome as the sun shines bright behind them and makes it more of an aesthetic kind of picture.

You can use this in any other day the there is sunset as well or any day really that just lights bright behind the subject, that when you take the picture against the lighting you’ll get your fill flash photo.