"DEFRAG" is used to defragment your hard drive
"DEFRAG C:\" will defragment the C: drive, you can chenge the drive letter if need be.
To break out of the DEFRAG simply press "CTRL and C" on the keyboard
DEFRAG can take some time and you usually need to have at least 15% of your hard drive space free for it to work correctly.
"COPY /?" will give you further help when using the command.