RegPack packs the regsitry. RegVac has the identical function. It is called Pack Registry.
When entries are added to the registry, they are written to the end of the file instead of being inserted in the file where it should actually be. When an entry is deleted from the registry, it is simply blanked out so that the space that the entry occupied is still there, it is just a bunch of 0s. When you look at the registry, you do not see any of this, but it is still there.
The hard drive works similarly.
If it gets bad enough, it can affect the performance.
RegPack cleans up the mess. It reads all of the entries, deletes the registry file and then writes them all back in order and without any blanks. None of the actual entries in the registry change. Only their location in the registry file changes.
RegVac on the other hand, goes through the registry and cleans out bad entries.