"Fill" flash means that rather than the flash behaving as the primary source of light, it's used to "fill" the shadow areas. This means the flash is typically set to be a bit weaker than the main source of light.
On cameras or flashes that allow you to control the amount of power used by the flash, you can usually dial the flash back to "-1" Each full "1" value means either a doubling or halving of the power level... so "-1" means you want the flash to fire at 1/2 the amount of power the camera would normally have used. This means the scene is lit by about 2/3rds natural light and 1/3rd flash. The result is an image where you still have shadows (so the image will look natural)... but the shadows are gentle rather than harsh-deep shadows.
If you have a camera that does not allow you to control the amount of flash, just set the camera to force the flash to fire. It will be an improvement over the deep shadows, but you may not notice much of the naturally occurring shadows.