The word 'dump' is usally used in the IT-language for an exact, untouched copy, e.g. a memory-dump is a exact copy of your computers RAM (or part of it) safed to a file for future reference.
When dealing with optical media, unfortunatly there are 1000s of ways to make a copy of it. You could just copy all the files from a cd to your hdd and install/run the game from there.
You could burn the files back to a blank disk and that could be considered as a copy as well. You could delete unneeded files, alter files to crack protections, edit media-files to reduce size etc and burn them back to a disc. Again, that could be considered as a 'copy', but in fact that would be a custom version of the source media, also called a 'Rip'.
To get an almost 1:1 copy, you could use your favorite burning-tool and create an image from your orginal disc. But since the devoloper of the cd-standard never though about the need for bit-identical copies, you will probably always get different results. Esp. when dealing with audio-tracks two users will never get the same result using normal image-tools. Google for 'pregap'.
This is where the redump-project comes in. They developed a guide to make sure you will get consistent results when backing up a disc.
You now might wonder with it's called 'REdump'. I don't for sure, but almost all discs has already been dumped in the past but must be dumped again (aka redump) from the original disc to fit the projects quality guidelines.
Sharing of the actual disc-images is not part of the project, but there are some side-projects providing bittorrent-downloads.