GRUB stands for GRand Unified Bootloader. Its function is to take over from BIOS at boot time, load itself, load the Linux kernel into memory, and then turn over execution to the kernel. Once the kernel takes over, GRUB has done its job and it is no longer needed.
GRUB supports multiple Linux kernels and allows the user to select between them at boot time using a menu. I have found this to be a very useful tool because there have been many instances that I have encountered problems with an application or system service that fails with a particular kernel version. Many times, booting to an older kernel can circumvent issues such as these. By default, three kernels are kept–the newest and two previous–when yum or dnf are used to perform upgrades. The number of kernels to be kept before the package manager erases them is configurable in the /etc/dnf/dnf.conf or /etc/yum.conf files. I usually change the installonly_limit value to 9 to retain a total of nine kernels. This has come in handy on a couple of occasions when I had to revert to a kernel that was several versions down-level.
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