Nothing, at least as you and I understand it.
You are either MANDATED to have health insurance or you pay a PENALTY if you do not.
Neither is a tax.
But instead the word 'tax' into the Supreme Court verbal transmogrification machine, and suddenly mandate/penalty is a tax.
It no longer has to walk like a duck and sound like a duck to be a duck. It can be a turkey or a penguin and still be called a duck by the time SCOTUS gets done with things.
corruption, pure and simple