This is the home directory of the System Administrator, 'root'. This may be somewhat confusing ('root on root') but in former days, '/' was root's home directory (hence the name of the Administrator account). To keep things tidier, 'root' got his own home directory. Why not in '/home'? Because '/home' is often located on a different partition or even on another system and would thus be inaccessible to 'root' when - for some reason - only '/' is mounted.