HDR, short for High Dynamic Range, can bring out the details of both the highlights and shadows in your shots. Use HDR when capturing your subject in high contrast lighting, or when shooting in lowlight conditions.
To find out more, go to the Overview page in this URL: http://www.htc.com/tw/help/htc-one-x/#overview. Under How-tos, see Camera > Using HDR. You can also watch the video, "Capturing photos in backlight conditions".