A codebook is a GitLab or GitHub repo containing code and Markdown (EzMark) notes that you keep for yourself as you learn different things. The idea is that this isolates your learning content from actual content from other projects. You can make your repo private or public.
If you choose to make it private they don't even see it (unless you invite them as collaborators).
If public you can explain in the
README.md file of your codebook that it contains random things you are messing around with and learning. That way people who happen upon your content—and they will—won't find a bunch of repos that don't having anything of substance in them.
Don't confuse a codebook with a codeboard which is something I maintain and update for each individual lesson.