It seems to leave out some important information. Eg. It refers to HEAD many times, but never seems to explain what this is? I can find out what this is easily

It seems to leave out some important information. Eg. It refers to HEAD many times, but never seems to explain what this is? I can find out what this is easily enough elsewhere, but from my immersion so far I feel that it should be explained. I really like the site though and have bookmarked it for later, and I will definitely try out the rest of it. Well done and thanks.