When something gets stored/written to the drive, it goes wherever it can. So if you have a document that's 10 blocks large, and there's a 6block open spot, thays where it goes, the other 4 going someplace else. That happens with everything. Do it enough times and your drive gets fragmented. Little bits of info all over the place, not combined in one nice neat package.
Optimization or Defrag is used to take those bits of info, and combine them into one package. It sorts and organizes all your files into big blocks of data, so windows only need look in one spot for the file, not spend 10 minutes hunting down a million different blips spread willy-nilly.
If your drive is reporting 0% fragmented, it's already got everything sorted. If...