GRUB Linux is a bootloader, a special type of software that loads an operating system when a computer is powered on. A bootloader is responsible for loading the operating system and providing basic hardware management services such as selecting which disk drive to boot from, selecting an operating system to boot, and providing low-level hardware configuration. GRUB Linux is the most widely used bootloader in Linux-based systems, most commonly used with the popular boot system "GRUB2".
GRUB stands for Grand Unified Bootloader, and it is a free open-source code version of the GNU GRUB boot manager developed for Linux and other operating systems. As a bootloader, GRUB provides the necessary support for booting and managing a wide variety of operating systems using the same underlying machine code. The GRUB bootloader can select from multiple bootable operating systems, allowing you to choose between Linux, Windows, or other operating systems at startup. GRUB also enables you to adjust which kernel to load and can be used to rescue and repair failed operating systems.
The GRUB bootloader is an important piece of technology for any Linux-based system and is strongly recommended for all Linux distributions. The biggest benefit of using GRUB is that it allows you to select and boot multiple operating systems and provides failsafe mode to help users repair or recover systems when they become unbootable. GRUB is highly configurable and provides a lot of features such as grub-mkconfig, grub-install, and grub-reboot.
For more information about GRUB Linux, check out this excellent guide: https://www.howtogeek.com/107614/what-is-grub-and-how-does-it-work/
