So what that means is that we're limited in any atom to having two electrons per orbital, right, because for any orbital we can either have a spin up electron, a spin down electron, or both.

Beyond spintronics, the discovery that DNA has a strong effect on electron spin suggests that spin interactions could also play a role in some biological processes.

So by parallel we mean - they're either both spin up remember that's our spin quantum number, that fourth quantum number.