Its the genital nub on an ultrasound.. shows like a little white willy that all babys have in early development. They say if its laying horizontal to babys spine its gonna be a girl n if its sticking up at a 30degree angle to the spine it'll be a boy.
It's a way to supposedly predict the gender of the baby by looking at the ultrasound, about six weeks before you would otherwise just see on the ultrasound. You can find it by googling the phrase "nub theory."