High Dynamic Range.
The spread of RAW for a single shot is about 5 stops, with high contrast scenery having a range of about 8 stops & the human eye having a 10 stop range. HDR helps you combine several exposures of same shot to aid the inclusion of the wider stops in your shot.
Nicked from CS2 help
The dynamic range (ratio between dark and bright regions) in the visible world far exceeds the range of human vision and of images that are printed or displayed on a monitor. But whereas human eyes can adapt to very different brightness levels, most cameras and computer monitors can capture and reproduce only a fixed dynamic range. Photographers, motion picture artists, and others working with digital images must be selective about what’s important in a scene because they are working with a limited dynamic range.
High Dynamic Range (HDR) images open up a world of possibilities because they can represent the entire dynamic range of the visible world. Because all the luminance values in a real-world scene are represented proportionately and stored in an HDR image, adjusting the exposure of an HDR image is like adjusting the exposure when photographing a scene in the real world. This capability lets you create blurs and other real-world lighting effects that look realistic. Currently, HDR images are used mostly in motion pictures, special effects, 3D work, and some high-end photography.