The HUD is a "Heads Up Display". It provides information such as heading, airspeed, altitude as well as localizer and glideslope indications. I also believe that there is some sort of flight director displayed as well. These have their presentation focused at long distance so that when you look at the info on the hud your eyes don't have to refocus (which can take a second or two) on the runway. Usually they are used in aircraft which are approved for Cat II and III approaches where landing visibility is just a few hundred feet.
Hope that helps