Defragmentation

Defragmentation (or defragging), is the process that eliminates fragmentation in file systems. It does this by moving blocks around the screen, or in some modern systems, changing colors of slimmer, more attractive blocks. It rearranges file data clusters in chronological order, and relocates them to the 9th level of a custom DOOM II level that you made your sophomore year in high school.

Reasons to Defrag
Defragging your hard drive only takes about five minutes, and is a recommended security measure to ensure the safety of personal information such as social security numbers, credit card PINs, and your Match.com password.

Basically, it's like therapy for robots. A deep tissue massage for your RAM, if you will.