Ok an atip hider is a little tool that hides the physical characteristic of a CD-R disc. These atips are impossible to get off the disc.
When SafeDisc 2.4 came out, the developers had introduced an ATIP check. This is still in the current SafeDisc versions.
So the program basically trys to search for an ATIP on the CD-R disc and if it finds one, the program won't start.
BUT, this only affects people playing their backuped game from a burner, not people playing from just CD readers or DVD readers.
To get around this problem, the makers of Alcohol 120% and CloneCD made an ATIP hider tool to go with their software. When you enable this ATIP hider, CloneCD or Alcohol 120% hides the ATIP on the CD-R disc from the program, allowing you to play on a burner.
BTW, only SOME burners can copy a SafeDisc game correctly, so if you don't know what your burner is chances are you don't have a SafeDisc 2 Killer.
Try to find out your burner.