https://www.quora.com/What-is-a-repacked-game?share=1
A repack is simply what it says it is. A repack can be of any computer program or software, which includes video games.
Basically, you take all the setup, configuration & data files which make up the core of the software and 'pack' them up into a single custom installer program. This is often very highly compressed in the case of video games as they particularly have very large data files and it's done so to save space. But this also means that it can take the installer much time to 'unpack' it which in turn depends on your *****U speed and system memory.
In video game piracy, a repack is a complete repackaging of the game files along with the crack/fix required to run the game DRM free.
Usually, repackers will take the legit data files of the game/software, apply a crack by some other scene group and repackage the whole thing into one complete package.
So when you run the the repacked game program, it will just 'install' (extract) the core files to the specified folder and you can run the game right away after install as the DRM had been fixed already.
This, by the way isn't exactly legal though.