A saying among young, mostly American, men in the late 20th century and early 21st century to remind them that "Your male friends are more important than anything female."
John meets Mary at the local bar and starts to take an interest in her. He later discovers that his best friend Bob has been trying to get with Mary for some time. John, at this point, should back off because Bob already has laid claim to her. John is following the rule of "bros before hoes".