There might be the situation, that your root filesystem runs out of space, so it has to be expanded. With filesystems other than the root filesystem this could easily be done for instance with gparted. With the root filesystem this is not possible, as gparted could only expand filesystems that are not mounted, which is impossible with the root filesystem. The only way to enlarge the root filesystem is to boot from a kind of rescuesystem, Live-CD or the like, and then to expand the root filesystem. The following tutorial describes, how this is achieved by using Systemrescue-CD...