“In image processing, computer graphics, and photography, high dynamic range imaging (HDRI or just HDR) is a set of techniques that allows a greater dynamic range of luminances between light and dark areas of a scene than normal digital imaging techniques. The intention of HDRI is to accurately represent the wide range of intensity levels found in real scenes ranging from direct sunlight to shadows.” – Wikipedia
What does that mean? A camera has a limited amount of ‘steps’ between the brightest white and the darkest black, and spans about 9 f-stops. The human eyes perceives up to 24, so even if you don’t know what f-stops are, you can see that there’s a big difference.
In HDR photography, you combine multiple exposures of the same scene, thereby gaining a wider range of detail than would be possible with just one exposure.