As good as today's cameras are, they still have their weaknesses and occasionally still need a helping hand from us to pull off a properly exposed shot. Especially when it comes to certain lighting conditions. These problems usually happen when your shot is in contrasting lighting conditions. Usually it's when a shot contains a strong light source in the background.
Take a look at the example below: You can see the cameras Auto Mode, incorrectly exposed this picture because of the strong light coming through the background. The cameras AutoMode thought that the scene had enough light. As you can clearly see the main subject was badly under exposed, almost dark enough to produce a silhouette.
Without Fill Flash
Obviously the issue with this shot is that there is just not enough light from the front of the subject, to light up the subject with the shutter speed selected by the camera.
While there are a number of possible solutions to this problem, one of the easiest is to override the camera and force it to shoot with a flash. This is commonly referred to as the Fill Flash Technique. Fill Flash is used to supplement existing light in a scene - it’s generally not the primary light source, but fills in light where natural light doesn’t go. Here's that same shot with a fill flash:
With Fill Flash
In addition to helping with backlit subjects fill flash helps in more subtle ways also by helping to eliminate shadows cast by facial features (under eyes, noses, chins) or under hats - especially when light is shining down from above. It also could be used to add a special little sparkle to the eye of the person you’re photographing. Producing those lovely catch highlights.
Most of todays cameras have built-in support for Fill Flash. In most cameras, there's problably a little button, with a picture of a lightning bolt (flash) on or near it. But be sure and check your camera’s owners manual on how to activate your cameras Fill Flash mode.