"High dynamic range imaging (HDRI or HDR) is a set of methods used in imaging and photography, to allow a greater dynamic range between the lightest and darkest areas of an image than current standard digital imaging methods or photographic method"
Basically you take 3 photos of the same thing at different exposes, 1 high , 1 medium and 1 low, so they focus on dark areas, medium areas and light areas, then you bring them all together :)