Fill flash simply append an extra burst of light to balance out the light in your photo and make it more visually alluring. It is always best to use fill flash when sun isn’t directly on the subject or it is too dull. Using fill flash opens up your creative possibilities as a photographer you will come to find out that creative photos don’t always line up with the light available in the scene. You must setup your camera to expose for the background before you setup your flash to expose for your subject. In the evening you will want to expose your background with a slower shutter speed and a more open aperture this lets as much light in as possible. Bouncing a flash of a surface can be really helpful, when you bounce a flash off a surface before it reaches your subject, the flash spreads out appears more like it has come from a faraway source. The effect of bouncing a flash off a surface makes it appear more natural.