The isolated girl in a group of socially acceptable or popular girls who is, very noticeably, unattractive. A group of attractive or popular girls will often adopt a fat girl, an "ugly" girl, a socially awkward girl, etc. They usually do this in order to make themselves feel better about being popular, or to look better by comparison. In most cases, the H-factor is a fat chick, but not in all. The H comes from the term "Holder".
"Hey man, why are those babes hanging out with that one fat chick? She totally stands out."
"I don't know. She must be the H-factor."
"Amanda is always hanging around hot girls. She wouldn't have many friends if she wasn't an H-factor."