To rearrange disk clusters to be contiguous, details ...
As data is being continuously copied and deleted from your Hard Drive, the data often becomes separated from the rest of its common files, slowing disk accessing times. Defragmentation places all the data in its correct place on the Hard Drive.
The process of rearranging the files on a hard disk so that all the files are as contiguous as possible, and that the remaining free space is also contiguous.