GRUB - GNU GRan Unified Bootloader
The name itself indicates the function of the GRUB. In simple language it is the platform on which you can load many OS like windows, ubuntu, redhat and other OS which uses the linux kernel.
When do you get this GRUB loading error?
This error occurs when the OS usually an open source like Ubuntu is Installed along with windows or other OS.
What is the Solution?
Try reinstalling the linux OS properly using the live CD or using the live CD you can edit the GRUB loader.