Tries to restore grub on the MBR automatically for you.
'Boot Gnu/Linux' will boot your operating system using your kernel and initrd.img files. This method of booting is very similar to a 'direct boot'. This is the most reliable way to boot up your (disabled) system. Your system should boot using this method regardless of whether Grub is installed in your MBR, and regardless of mistakes in your menu.lst file.
Boot Gnu/Linux Indirectly
'Boot Gnu/Linux' will give you a 'configfile boot'. In other words it will find your Grub menu and will present you with that so you can boot from it. This is useful when your problem is you do not have Grub installed to MBR for some reason, and you need to boot anyway. You can also boot any other operating system from your normal Grub menu. This will not be useful if your problem is caused by an error in your present Grub menu. You will still have the same error, so your system still won't boot.
'Gnu/Linux (Advanced)', is a shortcut to your Super Grub Disk's Gnu/Linux Advanced menu, for those who have more than one hard disk, or more than one Windows or Linux operating system, (multi-boot).