To defrag, that's what it means.
The way Windows stores files caused the files to be in bits and
pieces (aka fragments) everywhere.
When you access it, Windows will piece them back, like jigsaw
puzzle. If those fragments are all over the hard disk, it will take
a longer time to piece them back. So defragmenting files aims to
move the file fragments back to one whole piece, rather than being
everywhere on the hard disk.
This improves speed and disk space for some, because file fragments
take up some hard disk space as well.
