CD players may use three or more laser beams to stay on course on the CD. The CD is kinda like a vinyl with a continuous spiralling track that starts from the inside and goes outside. One beam reads the information in the middle of the track, the two other beams on both the left and right side of the track stays steadies the head (ie: like a rail road). Small radial scratches are mangeable. The worst is a scratch that parallels with the track because it will disrupt data readability completely and the tracking mechanism may fail beyond correction. Further, these fixing kits are suspect because they try to fill in the scratch with a different material that may improperly affect the refraction properties associated with the laser beam.
If the data is corrupted and detectable, the cd-player may try to apply error correction (some funny math-bit manipulation stuff here). If the corruption is really bad, then it wll try to interpolate. Expensive cd players will try to read ahead and determine the smoothest interpolation. Cheap CD players will try to read somewhat ahead and do a liner interpolation. Older cheap CD players give up and just put a blank signal their (annoying and dangerous to speakers).
The information on the CD audio is scrambled time-wise so a big scratch will disperse the "energy" of the error across a chunk of time reducing the noticeablility of the error. In other words, a single gash on the CD may produce tiny periods of errors dispersed in time. Without this, that big gash may produce a notiecable error over a single period of time.
So what MacDef said is true only up to the point the CD player cannot manage the errors. Of course, if you listen to a perfect CD copy next to a severaly scratched copy, you may notice a difference (I actually have exprienced this too).