The defragment function is used to defragment the filesystem on a partition.  To defragment a partition you must first select it in either the partition diagram or the partition list (when the selected item is empty space, this function is disabled).  Only some filesystem types (FAT for example) can be defragmented, so this function will only be enabled if the partition is of a supported type.  Before defragmenting a partition, you must commit any outstanding changes using the write changes function, or else undo your changes using the undo function.  Once a defragment commences the data is physically rearranged on disk, so you cannot subsequently use the undo function.