HDR stands for High Dynamic Range; a single photograph composed of multiple exposures (known as exposure bracketing). It’s a technique of photographing a subject with multiple exposures to get the details in highlights and shadows.
The digital cameras photographers generally use have a very low dynamic range in comparison with the human eye. We can see details in highlights and shadows with our naked eye. The HDR technique is a way around to this problem. This solution has also extended the creative possibilities for the photographers.