Alright, I'll bite. What's ABO?
It's short for alpha/beta/omega. It's a kink trope very popular in certain American circles. I gotta say, I too I'm surprised it has reached Japan.
In typical ABO there are three biological-psychological classes to which a character can belong. Combine with the two genders (male and female) and you get a grand total of six sexual roles or patterns of sexual behavior. The 3 classes are Alphas, Betas and Omegas.
Alphas are usually dominant and able to impregnate any Omega. Betas are like normal humans and subordinate to Alphas. Omegas are natural-born submissive bottoms, with males who are self-lubricating and can become pregnant.
If you wanna know more, go here: https://fanlore.org/wiki/Alpha/Beta/Omega