Click under Error-checking, "Check now" to Check your hard disk for errors.
As noted in the margin:
Fragmentation makes your hard disk to do extra work that can slow down your computer. Removable storage devices such as USB flash drives can also become fragmented. Disk Defragmenter rearranges fragmented data so your disks and drives can work more efficiently.
Disk Defragmenter runs on a schedule, but you can also analyze and defragment your disks and drives manually.
If you are prompted for an administrator password or confirmation, type the password or provide confirmation.
To determine if the disk needs to be defragmented or not, click "Analyze disk".
After that: click "Defragment disk".