Defragmentation, or "defrag," is a procedure using this procedure you can make a computer perform faster, when run sometimes. In the end, files in a HDD can get fragmented, and defragmentation is the "process of consolidating fragmented files," according to Microsoft.

Clusters
Data in a HDD is saved in bunch of data known as clusters. A cluster is normally 512 bytes, and if a file is large enough to necessitate the use of more than one cluster, the clusters are not essentially stored next to one another.

Need to Defragment
As a computer HDD saved info randomly, data are linked with same file can be saved in multiple places when you save, change & delete file from the computer. Accessing fragmented files slows down the computer.

Disk Defragmenter Tool
Disk Defragmenter is a tool in Windows-based computers that reorganizes and come together fragmented files and makes the computer perform better.

Using Disk Defragmenter
The Disk Defragmenter tool is run manually by the user. Newer Windows OS have an option to set a plan for the tool, so that it runs automatically, according to the input schedule.

Manual Operation
You can run the Disk Defragmenter tool manually by right-clicking "My Computer" and chose the "Manage" option. Start the procedure by clicking on the "Defragment" button, after choosing the drive you want to defragment.