The AV help section is definitely the best place to get a good understanding of what criteria they use to rank a site.
However, they do on occasions include info that can be a bit misleading.
"The purpose of the "keyword" METAtag is simply to allow you to add synonyms -- words that are appropriate for what's on your page -- that describe what's there but that do not actually appear on that page. One of the best uses for "keyword" METAtags is for foreign translations of the main words on your page, so, for instance, somebody searching in French will find that page."
Unless your synonyms and/or misspellings are extremely unique, and AV has no other pages in their index that contain these words in the title or top of the page, you have virtually zero chances of showing up for them.
For example, if you do a search on AV for a term like "leather cap" you will find that although all of the top sites returned contain the word "hat" in their meta tags, none of them show up for a search on "leather hat." Although the two words are synonymous, AV will not return any synonym matches until it has exhausted all pages containing an exact or partial phrase match.
That makes the whole effort involved fairly useless. It usually won't hurt to add keyword tags, but plenty of pages rank well at AV that don't have any keyword tags at all.