When system fails to boot up, "Enable/disable rescue mode" allows the user to access the hard disk.
The Rescue system configured uses Rescatux. After enabling, boot it up, and you will need to find that the original hard disk first by typing command
You should be able to see hard disk devices for example. /dev/sda1 which should match up to your hard disk size ( ex. 50GB )
Then all you need to do is to mount the device to a folder. ex. /mnt by using command
sudo mount /dev/sda1 /mnt
Make any changes required as necessary
Disable rescue mode once you are done.