It refers to the image quality after compression.
Most popular DVD compression programs are what they call transcoders.
A transcoder accepts that the studio has done the perfect job of optimising motion vectors and certain other parameters and uses these as they are.
In reality studios don't care if it's fully optimised or not as all they're interested in is that the quality on DVD matches their standards.
DVD Rebuilder is a full re-encoder that redoes everything from scratch and fully optimises everything throughout the DVD.
It takes a bit longer but the improvement in quality for difficult sources is very noticeable.
If you're only interested in ripping 1:1 copies then I'd also recommend the free version of DVDFab as that really is all you need.