It's for helping disabled people to use a web site easier.
See:
http://www.w3.org/TR/WCAG10/
For levels and a checklist, see:
http://www.w3.org/TR/WCAG10/full-checklist.html
It's a long read, and the only way you're going to understand it is to
do all the reading yourself. It's a pretty big spec, but it's not all
that hard to get through. At the end of the day, you'll make better web
sites if you understand it and can implement some of the
recommendations there.
I don't care about handicapped people, so I just ignore it. Just kidding!