the "-" means to make the shell a login shell, that is, for whoever user the target is, it will read in the profile files like .bash_profile, .bashrc or whether the files are depending in the shell (for example tcsh would use .tcshrc and .login). This is not Fedora specific, pretty much any unix like Solaris will use "su - <username>" or "su --login <username>" where if the <username> option is missing is consider to be 'root'
Since the default is to consider the user target to be root when you don't actually type in the name,
means "switch user to root and make it a login shell"
A more verbose, obvious way to do this is:
see the "man su" for more info:
Change the effective user id and group id to that of USER.
-, -l, --login
make the shell a login shell