lets say you can set your camera to its proper ISO for a particular film
then you go outside with your camera and flash
you take a reading of lets say a person with the light from the sun hitting the subject at an angle
you take a meter reading and find that the sunlight side of the face reads lets say f16 at 1/125 of a second
If you set your flash so that it fires off f11 or f8 at 1/125 of a second at the subject.
then you would be giving a fill flash,
ie a flash that is not stronger than the main light (THE SUN) but slightly weaker . This flash will fill in the shawdow area of the subject.
basically it is a way of controlling the LIGHTING RATIO of a scene like this
Whole studio set-ups are based on lighting ratio and fill flash is an important consideration when taking a portrait.
I hope this helps