A photographic technique in which a flash (usually an electronic flash) is used to fill in the shadows in a high-contrast situation.
The typical use is for outdoor portraiture, where the strongly directional light of the sun produces deep shadows in the eyesockets, under the chin and under the nose.
A flash mounted on camera will nicely fill in the shadows.
method: the details depend on your photographic system (or lack thereof), but the idea is that the flash should be underexposed: this way the main light in the scene will still be the sun.
On a TTL flash it is usually possible to dial in exposure compensation - a good starting figure is 2 stops, but some people use 1.5 stops or even 1.7 (even though I suspect that they do that only to show off their SB28).
On an autoflash you have to lie about film sensitivity, and claim that your film is four times (2 stops) more sensitive than what it really is.
On a fully manual flash, you have to use your natural computer, and do the usual guide number calculations .
Some automated compact cameras will do fill flash by themselves when they detect excessive contrast in the scene.