Linux From Scratch is a series of (online) books that walk you through building up your own linux system from the ground up, from compiling the kernel to all the individual systems that turn the kernel into a functional OS.

It’s meant to be an educational tool to help people learn about what goes into making a Linux distribution and give you better knowledge of how to build software from source. Some people turn these systems into their own distributions or personal (I guess gentoo-like?) Linux installs