I will try to explain this the best that I can!
Fill flash is used to supplement existing light in a scene. It's generally not the primary light source (as a flash often is in low light situations) but fills in light where natural light doesn't go. It helps to eliminate shadows cast by facial features (under eyes, noses, chins) especially when light is shining down from above. It will also help to add a "sparkle" to the eyes of the person you are photographing.
In most point and shoot cameras the main button to look for will be the
one with the lightning bolt (flash) on or near it. Press this multiple
times and it will cycle you through the different options for flash (on,
off, automatic, red eye reduction are the main four). Some point and
shoot cameras even have a ‘fill flash’ option that ramps back the
strength of the flash a little which is ideal.