Make this simpler: GRUB is the main example of the Multiboot Specification, which allows a user to have more than one operating systems on their computer at once, and to choose which one to run when the computer starts.
GRUB is the reference implementation of the Multiboot Specification, which provides a user the choice to boot one of multiple operating systems installed on a computer, or select a specific kernel configuration available on a particular operating system's partitions.