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.
Late response, but I started writing Your Dream Girl's Harem some time ago, though I'm taking a break from it at the moment. That said, it has (reluctant) bi content pretty much from the start, so it might not be everyone's cup of tea.
"Reverse harem" name comes from the same logic as "reverse gangbang" which in hentai was the term for a guy getting fucked by multiple women or "reverse rape" which is a guy getting raped by a woman. The latter term especially is weird and creepy, but that was the naming convention used by Japanese porn fans a few decades back. Reverse harem is the more mainstream term, because there are plenty of non-porn works that use the term. Dating games, romantic mangas and such for example may use "reverse harem" as a category or to quickly let readers know what the content is about. "Reverse gangbang" term can also be found on Western porn sites as well, so I guess that term has spread as well.
