What is GRUB?
GRUB is activated after the initial BIOS boot. It is installed in a special place on disk and loads the kernel and root file system. Once complete, it will transfer control to the kernel.
Configuration
Configuration files live under /etc/default/grub and /etc/grub.d
The generated GRUB config file is under /boot/grub/grub.cfg.
Typically Grub will get updated through packages, such as when installing a new kernel, the config will get updated to ensure that it gets booted with the new kernel.
Kernel Boot
When the kernel is initially booted, GRUB is responsible for passing the parameters to the kernel. Some example parameters are cachesize or clarcpuid for disabling CPU features. Additionally init parameter can be sit to override what to be run instead of sbin/init.