When your player (Foobar, WMP, what-have-you) requests a song to be played, your CD drive selects the correct position on the CD, aka a track. (tracks don't physically exist, it's one continuous data stream) However, due to various things, your drive is not positioned perfectly at the start of the track. It may be a few samples (remember, a CD has 44,100 samples per second of audio) off. People have compiled databases of drives and their own offsets, or how far off the drive is. Using EAC (only program I know of that allows you to do this), you can tell it how far off your drive is, and it corrects for this when ripping or burning by telling it to move ahead or backward, whatever the case may be.
For instance, I use my burner for ripping, as I know what the offsets are. Reading is +685 samples off, writing is +6 samples off.
Now, will this make a difference? No. Don't believe anyone who tells you it will. Unless it's horribly (thousands and thousands) off, you won't be able to hear a thing. Think about it. If 44,100 samples equals one second of audio, than 685 samples equals about 1/64 of a second.
So why do I (and others) correct this? My theory is that we're all obsessive compulsive
However, it's also one more tiny step to remove any possibility of a loss of sound quality. My backups of my CDs are as perfect replicas as I can make them. And that makes me happy.
(-:Stephonovich