Wikipedia
In still photography and video, a matte box is a device used on the end of a lens to block the sun or other light source in order to prevent glare and lens flare. It performs essentially the same function as a lens hood and also mounts in front of the lens, but usually includes adjustable fins called French flags.
Another purpose of a matte box is to hold glass or plastic filters in place in front of the lens. Today, matte boxes are made for DV cameras and HD cameras for the same reasons as for film cameras. Some are supported by two rods that run the length of the camera, while others are supported by the lens itself.
A matte box may have a bellows, a rigid sunshade or both, where the bellows is positioned within the rigid sunshade, having a mask which may be adjusted forward or back to suit the angle of view of the camera system.