They rate what they think a character's chances are, and how much the people want the character. I have not really participated much, but I did voice my thoughts when they went through Geno's secound rating, which just so happened to be one of there most controversial rating days. Funnily enough, Geno is set to have a third rating day, due to how the situation has changed with so many popular picks happening.
It's at the top of the boards, right along the rules of the forum as well as general newcomer discussion. They aren't rating her now, but they have announced when.