it's when you pull out whatever is stuck in your belly button, most likely the excess cotton of one's boxer briefs. some like to call it the lint. Sometimes this 'lint' will naturally have some of your belly button hairs woven in the mesh.
Yeah, I've been wearing these boxers for 3 days straight now.....hey look, wait.....ahhh, there it is. Damn kid, look at the size of that belly jean