It is a program that defragments your hard disks and optimize it. As you save and erase files from your disk, the system will occupy and free clusters in your hard disk.
This free space will be used by new files, filling the spaces scattered in your disc, splitting the files in multiple blocks. When you read that files, your hard disk will move its head all over the disc in order to read those parts. That produces long waits and disc wear.
O&O Defrag Professional Edition packs file fragments back together to optimize your hard disk under Windows.