what is a rescue image? : I am trying to learn how to make my own os from here:http://wiki.osdev.org/Bare_Bones but somewhere there it says that I have to use the `grub-mkrescue` command. I was doing some research around internet to understand what does that command do and found in the [grub official documentation][1] this sentence: 

> The program grub-mkrescue generates a bootable GRUB rescue image

But I don't know what a rescue image is.

  [1]: https://www.gnu.org/software/grub/manual/grub/html_node/Invoking-grub_002dmkrescue.html#Invoking-grub_002dmkrescue