Defrag, short for defragmentation, is a computer process to reduce the amount of fragmentation on a file storage system.
The benefits are faster access to files to read and write resulting in an overall faster PC.
Fragmentation occurs during the normal process of running a computer when files are broken into pieces in order to fit onto the disk, which means that files become spread out over several locations on the disk.
Defragmentation reduces fragmentation by organising the contents of the storage device so that files are stored on sequential sectors on sequential rings of the disk.
There are several defragmentation tools available, some free, such as the built-in Windows Disk Defragmenter. There are also tools that can be downloaded from the internet that do the same thing, and some which keep your hard drive defragmented consistently over time rather than simply as a one-off process.
Defragmentation does not involve deleting files on the disk.