What are the hidden terms of health Insurance one should be knowing?

That would depend on your policy but they usually manage to find something they won't pay for. When I had bypass surgery I had met all my deductibles before they shipped me to the hospital where the surgery was performed but they still kicked things back to me saying they were the wrong HIPAA code or that it was part of another HIPAA code they had already been billed for. My feeling was that I was not the one who took a college course on HIPAA codes so why were they sending those to me? When they refused to do anything about that I sent them to my state insurance commissioner's office and miraculously they got paid. After about a half dozen of those the insurance company told me to stop sending them to the state insurance commission. I told them to stop refusing to pay them and I will. That was before Obamacare. If Obamacare is anything like Medicare they tell the hospital/doctor how much they are going to pay and you may be on the hook for the rest of the bill. The hospitals know that so they like to use outside  doctors/medical services that don't take Medicare so you are stuck with the bill yourself. Read your policy and make sure, if you can, the doctors and specialists are in your policy.