Oh, sorry, it's a fanfic community term. It just means the work involves a 'boy/boy pairing' or, in other words, a romance between two men. It's akin to 'm|m' ('male|male' or 'man|man'). Sorry for the obscure term, I'm still learning how itch likes to tag works like this! e: Decided after looking around a bit more on itch to change that to 'Gay Erotic Romance'.
Thanks for hosting the jam, by the way! I was hesitant to try and post a work to itch.io but the SGJ seems to have been the push I needed.