If we combine the comments from Hpshps and Rec, we are pretty close.
The file format redbook uses is call CDA. One feature of this format is self-generated clock. That means the clock is regenerated by looking at a limited run of the data stream in real time. If the data stream contains errors, the generated clock will be a little off and causes jitters.
Now if you make a copy, data streams are treated as files, the software can apply a much better error correction scheme to the data and burn a perfect copy of that data stream to CD-R. If high quality disc is used, you will get much lower error rate at playback and therefore much lower jitters.
You can hear improvement only if you use high quality CD-R and playback on relatively mid to low-end players. Most high-end players have large buffer to store data stream. They can apply extensive error corrections to it and regenerate the clock form there.