"root" is the system administrator account. Essentially, root has the ability to do anything to the system regardless of permissions. When you installed Red Hat, it asked you for an administrator password (I believe it had a yellow badge for an icon next to it). The password you entered there is the password for the root account. Later in the install, it asked you to create additional accounts. Any account you created in that step is a "regular user".
The best way to do this is to log in as one of the regular users you created and temporarily switch to the root account to do administrative tasks. You use the "su" command to do this, like so:
Password: <type root password here>
If successful, you'll get a prompt back. If not, it will complain about the password not matching. Once you're root, then edit the GRUB configuration file. Once finished editing, type "exit" at the command prompt to return to a regular user.