The fill light is a secondary source to the key, or main, light. The fill light fills the lighting 'gaps' of the key light, softening the direct shadows it causes and balancing the contrast, so it's visible, but subtler.
This filter increases the contrast between different shades of light and color, so the clouds in a stormy sky in your beach pictures are more distinctive. The filter also makes the blue of a cloudless sky or a tropical sea deeper and richer.
The ROM-based software in all PCs, digital cameras and other computer-based products include this micro program. Updates are provided regularly to address any bugs and improve the firmware's functionality.
This is an extreme variation of a wide-angle lens. A common wide-angle lens size is 20mm, while a fisheye lens is typically 8mm to 10mm. A fisheye lens creates a rounded image that shows the area in front of, to the side of and even a bit behind the camera.