In the real world, the scene may contain some well-illuminated areas as well as some areas that are in the darkness. The dynamic range of the scene, or the radiance ratio between the brightest and the darkest spots, can be quite big – it can span several orders of magnitude in some cases. This is a lot wider range than the range that our digital camera can adequately capture, and that our monitor can adequately display. The areas that are too bright for the dynamic range of our camera are going to be saturated on our image, and the areas that are too dark will be under-exposed. In both cases, all the information about these areas will be lost in the image.
A way to solve this problem is to combine different shutter speeds of our camera. Smaller shutter speeds can give us good information about the bright areas (because we won't have saturation), and longer shutter speeds effectively capture the dark areas. So by alternating the shutter speeds and by combining the two images into one in real-time, we can achieve higher dynamic range for digital video cameras.