In general "Upconverting" "UpSampling" or "Upscaling" is a process that mathematically matches the pixel count of the output of a standard or non-hi-def signal (such as standard DVD) to the physical pixel count on an HDTV, which is typically 1280x720 (720p) or 1920x1080 (1080i). So it gives you a better picture, some players are better than others as some tend to soften picture in the process. Please dont get confused and think this will give you Hi-Def quality f a HDDVD or BluRay player...but it can be much better.
