TTL stands for Through the Lens and fill flash is a technique
often used in portraits to fill in shadow. Light on a subjects face
will match the exposure required for the background. The camera
will vary the amount of light from the flash unit. Very useful on
sunny days when you don't want your subject squinting into the sunlight.
Correct fill flash will only be enough light to fill in the shadows
without blowing or washing out the subject. The eyes are said to
be the pathway to the soul and a little flash or lighting will also
provide a pleasing glint in the eye. The trick is not to use too
much flash.