HDR is an acronym for High Dynamic Range. Essentially HDR is a photographic technique in which you combine a series of images into one photograph that wouldn’t be possible to create using nothing but traditional photography techniques.
The basic process requires you to take a series of at least three images at various exposures to capture the full range of highlights and shadows of the scene. You can create the images after the fact from a single raw file, or if you are really ambitious, you can use up to 9 different images to create a single HDR image. Take a look at these fantasy movie art inspirations.
You can find out more about HDR Photography on this amazing website called DailyHdr