HDR stands for High Dynamic Range
. In the context of photography, the dynamic range of a scene is the difference between the brightest highlight and the darkest shadow. In many scenes, the dynamic range found in nature is wider than what a camera can capture.
There are several ways to compress the dynamic range of an image. The one I used here involved taking a total of seven pictures at different exposures (so from too dark to too bright) and blending them together digitally. There's a lot of choice for how to treat it, from subtle to garish. I went for something in between on this one, rendering it a little outside of what would normally be expected.