Just enough emitted light from the camera‘s internal or external flash to supplement available light.
Fill flash is useful in situations where the subject is backlit and the camera only sees a silhouette of the subject, when there are harsh shadows on the subject cast by strong available light, or simply to supplement other light sources.
Fill flash can be found on both digital and film, point-and-shoot and slr cameras. Try experimenting with the fill flash setting, even in sunny outdoor shots. This is especially useful when shooting outdoor portraits as fill flash will soften shadows on faces.