If I understand it correctly, it's a female protagonist who has multiple males devoted to her. It's not exactly reverse, more gender-switched, but that's how the term is used, as far as I know.
Another possible definition of a "reverse harem" would be an individual person who is made to be a sexual servant for many masters/mistresses. It seems to me that a story about such a situation would need to include at least SOME degree of consent given by the individual to the group, or it would run afoul of the site rules.
