There are various programs on your computer that will update frequently. (They often will do this automatically) However, the updates don't always get filed with the main program - These are fragments.
When you defragment, the computer sorts thru the fragments and puts them where they belong. This helps the programs run more smoothly.
You can defrag your Windows system by clicking on "START", then "CONTROL PANEL", then "PERFORMANCE & MAINTAINENCE", then "REARRANGE ITEMS ON YOUR HARD DISK..."