Fill in Flash is when, let say you have a very bright background like the sun nothing is brighter am I right. Then you have shadows that hide people’s faces like from their hats or from and tall object and you really want to see their face. That is when Fill in Flash comes in. What it is in the simplest terms is a "flash" that was so hard to explain you don't even know. But seriously it help bring out faces and people and any random object that you want to take a picture of. I kind of explained when and why it is useful already, because you want to see peoples face and other things.
My photos are kind of examples but not really. The top two yes they are because it show what I have tried to explain in the paragraph up there. You can barley see me in one of the photos and in the other with the flash you can see me but it's not like any other normal flash picture you can still see the background and me at the same time. As for the one of my partner I could have took better pictures, but you can tell what happening. Her face is covered by the shadow of her hand and the flash brings it out more and sort of fades away the shadow so her face is more visible. Until next time reader, bye.