A durable power of attorney for health care refers to a legal document of which someone other than you makes your medical decisions in cases where you are not able to do it. This person does not have to actually be an attorney. You can appoint anyone that you wish.
The person who acts on your behalf is called an agent, healthcare proxy, attorney-in-fact, patient advocate, etc. It is often seen that patients who are critical are not able to take their medical decision. This could be because lack of understanding of medical procedures and terms or the critical situation itself. This is where the role of durable power of attorney for healthcare prevails.