Wiki
How the product actually works, written out rather than summarised. If you are looking for the list of checks themselves, that is the checks page - generated from the same catalogue the scanner runs, so it is never out of step with it. This explains the reasoning the list cannot.
Scoring
How the health score works
What the number out of 100 is made of, why one broken robots.txt can cost more than a dozen missing alt attributes, and what makes two scores comparable.
Why your score changed
Four things move a health score, and only one of them is your site. How to tell which one it was before you go looking for a change nobody made.