Here is the video version, if you prefer it:
Boot loader’s job is to start the kernel – that is, to load the kernel into memory and start the kernel with a set of kernel parameters. (Ward, 2014) Kernel parameters are text-based parameters that tell the kernel about how it should start. Boot loader also selects among multiple kernels, switches between sets of kernel parameters, allows users to meddle with the kernel parameters and kernel images and provides support for booting other operating systems.
One of the most popular boot loaders is GRUB, short for Grand Unified Boot Loader. So, if you ever see some errors related to GRUB, you now know: “Oh, it’s my boot loader that’s the problem”.
To be honest, I never had to deal with boot loader problems, but I imagine they must be bad. May the force of Google help you in your quest to resolve those.
Thank you for reading!
Ward, B. (2014). How Linux Works: What Every Superuser Should Know (2nd ed.). No Starch Press. Pages 97-98