It means High Dynamic Range if I remember correctly. It basically sort of.. combines all the lights and darks together of the various images so you can really bring out a lot more information in the image, assuming it's not "too dark"..

So basically if you ever feel like your images are kind of washed out, or you're not getting as much color as you'd like, combining numerous different images together gives you a much wider range of color to work with.

Plus for some weird reason HDR shots often look more lifelike, not always.. But.. They often tend to make images look much more vivid and real. Hope that helps, I suggest googling for more information.