Okay, so sometimes adults have romantic and sexual feelings for each other. That's normal and okay, but sometimes they want to keep it a secret from other people. When two men have romantic and sexual feelings for each other and don't want anyone else to know, they might be on the "down-low".
This means they keep their feelings and actions private and don't tell other people. It's like a secret club just for them. It's important to remember that being on the down-low isn't necessarily a bad thing, but it's important for people to be honest and open about their feelings and actions with their partners and anyone else they may be intimate with.