What is Fill Flash?
Fill Flash is a technique used to brighten a subject with a flash when the main subject is underexposed, due to a strong back light.
A Little More Detail:
Suppose you’re shooting portraits outdoors on a sunny day, and the bright background is causing the subject to look dark and underexposed. Setting the exposure
to the background and applying a flash to the subject allows you to properly expose the entire scene. This technique is called Fill Flash.