Fill light is soft (softer than the main light) light that is used to expose the features of a subject that might be hidden by the shadows created by the main light (the main source of where the light is coming from, ex: sunlight, light bulb, etc.). In portrait photography fill light very often comes from a reflector.
|This is a light reflector. It is positioned at an angle|
that will help redirect the light (that is not heading towards
the subject) towards the subject. This will help light up
multiple sides of the object.
A reflector can be many things: a wall (preferably light colored walls, dark colors absorb to much of the light unless the subject is right against the wall), a table, a ceiling, and many other surfaces that redirect light in a desired direction.
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Some photographers use flashes in order to create fill light. Flashes can be positioned in multiple angles, and some can even be shot independent of the actual camera. One example of using Flash as fill light is when I took the picture of the baby below. This picture was taken under a 10x10 beach tent at night. I did not want to much light coming from one direction to hit the babies face so I angled the flash directly upwards. By doing so, the light from the flash bounced off the tent's ceiling and so the entire tent became a huge light box. By examining the picture you will see little, too none, notable shadows that are over powering (the background is dark because I took this outside during the night and so the amount of light inside the tent drained out the visible objects in the background, and a low f/stop also helped).
Bounce light is VERY useful, especially during portrait photography. So make sure to be aware of it's usefulness and how to best control it. Observe your surroundings for possible "reflectors" (i.e. walls, tables, flat surfaces) and be creative with light sources and when to take pictures (refer to my "The Golden Hours" post).