Combining multiple exposures of the same image to increase the dynamic range (The range of the darkest area with detail up to the lightest area with detail). All digital cameras and film have a certain 'dynamic range', so if you were to correctly expose for the ground, but also have the sun in the shot, the sun and sky would likely be 'burnt' out. Ie it would be too bright and beyond the possible dynamic range so any detail would be lost in that area and it would just appear white. so one way to overcome this is to use HDR (High dynamic range) to capture detail across a wider range.