Hi Polly :)
The nub theory is basically a supposed method of predicting baby's gender, which can be done with the dating scan (at around 12 weeks)
The theory is that although baby's genitals aren't fully formed yet, the tube that will eventually become the genitals looks different depending on the gender. Supposedly a girl nub will be at a lower angle in relation to the spine, and a boys will have a bigger angle, so it appears to point up more.
It's often used in conjunction with the "skull theory" which is based on skull shape, but I don't know much about this one! If you Google it, there are lots of handy pictures to compare to 