'Fill-flash' is usually required when one is confronted with abnormal lighting conditions, where a resulting photo may cause dark shadows to appear over the frontal or facial areas of a subject. These conditions are generally brought on by having the light source ahead of the camera lens or behind the subject.
Problems of this nature can also come about when photographing your subject in a snowy field or on a white sandy beach.
Photography taken in this manner, against the light, is known as 'Backlit Photography' and/or 'rim lighting', where the subject is being lit from behind.
When you look at an object that is lit this way, your eyes will automatically adjust to balance any adverse lighting difference, to a point. But your camera cannot cope passed certain levels of brightness, or shadow. If it sees an area behind the subject that is brighter than the remainder of the scene, its inbuilt light meters will read off the brightness in those areas and exposure the scene accordingly. This could then result in an image with a properly exposed background, but your subject is like to be in dark shadow. As well as anything else used as features in the foreground.
If taking photos of a subject in bright overhead sunlight, shadows will again be a problem under facial features and especially if the subject is wearing a hat or cap. Fill-in flash can also remedy this situation.
So, without going into bracketing and exposure compensation, fill-in flash is probably the most common way of treating this problem, as it balances out the differences between the bright areas and the shadows.
Reflectors are also used for the purpose of bouncing light back into the frontal areas of subjects. (Click here for more on reflectors and how you can easily make your own).
So, it is a matter of assessing the lighting conditions and knowing well beforehand, if they are going to affect the shot or not.
A lot of new cameras today have fill-in flash as an added feature and for those who were not sure what it was used for, well now you know!
For those who do not have this feature on their cameras, you may have to manually activate flash whenever the need arises.
If your camera has a hot shoe, you can fit an external flash unit and one that operates automatically TTL (Through The Lens), will offer just the right amount of foreground brightness you need for your subject, but also have a manual switch, for when you want extra light or less.
Today’s modern flash units swivel around as well as up and down so they can be bounced off a nearby white wall or ceiling and they can come fitted with a light diffuser to soften the light that is being directed to your subject, giving a more flattering look. And for even better versatility, you can also get a sync lead as a link between your flash and your camera. This allows you to hold the flash unit with a free hand while the camera is mounted on a tripod. Or as an added optional bonus, you can attach the flash unit to another tripod if you wish.
If on your camera, auto settings are not available, for fill-flash, the aperture setting flash needs to be set 1-2 stops smaller than with normal non-flash settings.
It is better to use subtle flash for fill-in - otherwise, there is a danger that double shadows will be produced, and the fill-in might impact on other subtle lighting effects that were wanted from the originally lit scene.
For example, in modelling, where lighting has already been set up to enhance or soften a model's features, care needs to be taken with fill-in flash - or it could completely ruin the overall soft, flattering effect.

The key to using fill-in flash and natural light together - is to keep the flash subtle. If an end photo has evidence of flash light - then too much fill-in flash light has been used, and such shots will look very artificial.
The aim is to only use an absolute minimum amount of fill-in flash - in order to correct excessive contrast and shadows.