It's kind of a loosely defined shared universe in (mainly) erotic fiction, and is also known as ABO, because in this universe, both male and female genders are further divided into three subgenders: Alpha, Beta and Omega. While Beta individuals are (in most cases) just like people in our world, Alpha and Omega individuals both have glands that produce a pheromone that is irresistible to the other subgender. Specifics are up to the individual authors, but in general, Alpha individuals tend to be very dominant while Omega individuals are very submissive.