What Does Fill Flash Mean When Talking About Photography?

In photography fill flash is light, produced by a flash unit which fills in a darker area of an image. Fill flash is not intended to overpower the main light, but bring out the detail lost in shadows. Fill flash is particularly useful in situations where the image subject is backlit by bright light, for example shooting a subject with the sun behind them. Without fill flash the dynamic range of such a scene would be to high to be captured in a single image, causing the subject detail to be lost in shadow. the use of fill flash lowers the dynamic range of the scene allowing more detail to become visible.